Quality explain
# What does this PR do?

This PR reorganizes the quality checks by splitting the current `make quality` in two:
- `make quality` is the check on code style
- `make repo-consistency` contains all other general checks on the repo

Similarly in the CI checks, the old `check_repository_consistency` check (that tested links used in the library are on S3 back in the pre-git repo days) performs the checks of `make repo-consistency`, while `check-code-quality` only focuses on the code quality.

In practice, nothing changes for the users of `make fixup`, but users of `make quality` have to remember two different lines:
`make quality` and `make repo-consistency`.

I also have drafted a doc page explaining all those changes and what each check does (for advanced contributors as well as team members), with TODOs of checks I plan to add in the future.

One last future plan I also have is to make the `make repo-consistency` command perform all checks and report all failures at the end (instead of stopping at the first one).
Thanks for the extended review @stas00 ! Will rename the page.