Add ReadTheDocs configuration file

[ACz-UniBi]

The ReadTheDocs project page of the Guidelines tells us that:

Your project(s) "guidelines" don't have a configuration file. Configuration files will soon be required by projects, and will no longer be optional. Read our blog post to create one and ensure your project continues building successfully.

How to add a configuration file for ReadTheDocs.

[ACz-UniBi]

Added and updated:

• https://github.com/openaire/guidelines/commit/7b7e212d2ceb7951e3e821b3bc3352728797e27a
• https://github.com/openaire/guidelines/commit/a09ff62e1116341c346ef5d4a9ccc43a9ae4c87c

Full change on the .readthedocs.yaml file at https://github.com/openaire/guidelines/commits/master/.readthedocs.yaml .