Config file: document usage of custom path for config file

[stsewd]

We have some documentation at https://docs.readthedocs.com/platform/stable/guides/setup/monorepo.html, but we should also mention that this is possible in the config file docs. There we mention that only a .readthedocs.yaml file is recognized at the root of the repo.

[Otavio72]

I will take this, my boyyyyy! 🚀