Update `Exercise anatomy` in `CONTRIBUTING.md`

[ryanplusplus]

• The tests now seem to be in test_{exercise}.c at the top level, with some test-framework/unity*.* files that seem fairly standard. This conflicts with the Exercise anatomy section of CONTRIBUTING.md (probably true of many tracks!).

Originally posted by @colinleach in #1006

This was messed up when the exercise structure changed when we transitioned from v1 to v2 (or v2 to v3... I can't remember).

[wolf99]

Could be a good first issue for someone... ? ☺️

[spinoDev]

Can I pick it? is their any update on the issue? Thanks.

[ryanplusplus]

Can I pick it? is their any update on the issue? Thanks.

All yours!

[spinoDev]

Hi! I started looking into this issue, but the current exercise structure seems to already match what's described in CONTRIBUTING.md. Am I missing a specific thing, or has this already been resolved?

[wolf99]

Was this actually resolved by PR #1248 @ryanplusplus ?

[ryanplusplus]

Yeah, looks like we forgot to close this when that was merged. Sorry for the confusion, @spinoDev!