introduce compound assignments for mathematical operators

The `flag-enums` exercise introduces the concepts of _compound assignments_ and has the following in its `after.md` file:

> The bitwise operators can also be used as [compound assignments][compound-assignment], which are a shorthand notation where `x = op y` can be written as `x op= y`:

We should introduce compound assignment in the `numbers` exercise's `after.md` document instead (the above text can remain in the `flag-enums` `after.md` file). In the `after.md` document of the `numbers` exercise, we should introduce compound assignments and use the ones with mathematical operators like `+=`, `-=`, etc as examples.

Furthermore, the `compound-assignment` concept in the `languages/csharp/config.json` file should be moved from the `flag-enums` exercise to the `numbers` exercise.

edit: updated