improve bit-manipulation concept

Currently, `bit-manipulation` concept is focused on `flag-enums`. The concept should be rewritten to introduce the student to `bits`, `bitwise operations` and anything else that would typically occur in an introductory text on the subject.

Assume that numbers and compound assignment operators, `+=, *=` etc, are prerequisites (and have been explained elsewhere).

Obviously, much of the material can be links to external documentation and arguably [this document](https://github.com/exercism/v3/blob/master/reference/types/bit.md).

Note that [this document](https://github.com/exercism/v3/blob/master/reference/concepts/bitwise_manipulation.md) will need to be updated with the new link to C#'s `bit-manipulation` but the landscape may have changed depending on what happens to `introductions`. It is a moving target but if it's not straightforward to change the link then just raise an issue and our leader will give us direction.