cleanup about.md files

Through a largely mechanical process the original _after.md_ document for each of the C# exercises has been split into _about.md_/_links.json_ files. Each component concept of the exercise is allocated a separate _about.md_ file.

The concept files need to be reviewed:

- TODOs need to be actioned
- A couple of "general reference" sections containing links may need to be removed (_links.json_ takes their place)
- Some text needs to be transferred to "approaches"
- Some closely coupled concepts need to be teased apart (e.g. arrays, for-loops, foreach)
- Exercise files need to be changed or new exercises need to be added where concepts appear to be introduced in an unconventional context (e.g. bitwise operations introduced in flag-enums)
- We need to find a way to compensate for the fact that concepts are no longer so closely associated with exercises. One of the particular advantages of the Exercism approach is that learning is structured round a framework of practical exercises - we should ensure we don't lose that in the case of concepts.

Ensure that the matching _links.json_ file is kept up-to-date.

I think that seeing how the concepts look as part of the user experience (which we should be able to do shortly with the beta) should help guide the final format of concepts.

The following concepts are affected:

- [accessibility](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/accessibility/about.md)
- [arithmetic-overflow](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/arithmetic-overflow/about.md)
- [arrays](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/arrays/about.md)
- [attributes](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/attributes/about.md)
- [basics](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/basics/about.md)
- [bit-manipulation](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/bit-manipulation/about.md)
- [booleans](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/booleans/about.md)
- [casting](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/casting/about.md)
- [chars](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/chars/about.md)
- [classes](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/classes/about.md)
- [compound-assignment](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/compound-assignment/about.md)
- [conditionals-ternary](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/conditionals-ternary/about.md)
- [const-readonly](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/const-readonly/about.md)
- [constants](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/constants/about.md)
- [constructors](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/constructors/about.md)
- [datetime](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/datetime/about.md)
- [datetimes](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/datetimes/about.md)
- [defensive-copying](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/defensive-copying/about.md)
- [dictionaries](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/dictionaries/about.md)
- [do-while-loops](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/do-while-loops/about.md)
- [enums](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/enums/about.md)
- [equality](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/equality/about.md)
- [exception-filtering](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/exception-filtering/about.md)
- [exceptions](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/exceptions/about.md)
- [explicit-casts](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/explicit-casts/about.md)
- [expression-bodied-members](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/expression-bodied-members/about.md)
- [flag-enums](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/flag-enums/about.md)
- [floating-point-numbers](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/floating-point-numbers/about.md)
- [for-loops](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/for-loops/about.md)
- [foreach-loops](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/foreach-loops/about.md)
- [generic-types](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/generic-types/about.md)
- [if-statements](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/if-statements/about.md)
- [indexers](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/indexers/about.md)
- [inheritance](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/inheritance/about.md)
- [integral-numbers](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/integral-numbers/about.md)
- [interfaces](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/interfaces/about.md)
- [lists](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/lists/about.md)
- [memory-allocation](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/memory-allocation/about.md)
- [method-overloading](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/method-overloading/about.md)
- [mm1.txt](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/mm1.txt/about.md)
- [named-arguments](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/named-arguments/about.md)
- [namespaces](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/namespaces/about.md)
- [nested-types](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/nested-types/about.md)
- [nullability](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/nullability/about.md)
- [numbers](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/numbers/about.md)
- [object-initializers](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/object-initializers/about.md)
- [operator-overloading](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/operator-overloading/about.md)
- [optional-parameters](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/optional-parameters/about.md)
- [ordering](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/ordering/about.md)
- [overflow](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/overflow/about.md)
- [parameters](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/parameters/about.md)
- [pattern-matching-constants](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/pattern-matching-constants/about.md)
- [properties](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/properties/about.md)
- [randomness](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/randomness/about.md)
- [readonly-collections](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/readonly-collections/about.md)
- [regular-expressions](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/regular-expressions/about.md)
- [resource-cleanup](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/resource-cleanup/about.md)
- [resource-lifetime](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/resource-lifetime/about.md)
- [sets](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/sets/about.md)
- [string-builder](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/string-builder/about.md)
- [string-formatting](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/string-formatting/about.md)
- [string-interpolation](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/string-interpolation/about.md)
- [strings](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/strings/about.md)
- [structs](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/structs/about.md)
- [switch-expressions](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/switch-expressions/about.md)
- [switch-statements](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/switch-statements/about.md)
- [throw-expressions](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/throw-expressions/about.md)
- [time](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/time/about.md)
- [timezone](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/timezone/about.md)
- [tuples](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/tuples/about.md)
- [user-defined-exceptions](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/user-defined-exceptions/about.md)
- [varargs](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/varargs/about.md)
- [verbatim-strings](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/verbatim-strings/about.md)
- [while-loops](https://github.com/exercism/v3/blob/master/languages/csharp/concepts/while-loops/about.md)