Create an intermediate README in docs/

With the README overhaul in #2358, the documentation is split up into multiple files in `docs/`.
Some repos place a `README.md` into this intermediate folder, showing a detailed description of what each documentation file contains, which is rendered nicely in the GitHub view.

After the PR in question is merged, we could think about adding one ourselves.

Review the README overhaul in #2358 and the documentation files currently in docs/ after that work is merged. Add a README.md in docs/ that gives a detailed description of what each documentation file contains and renders clearly in GitHub; completion means the intermediate folder has that overview.

Written by the indexing model from the issue text.