Add ReadTheDocs configuration file

[ACz-UniBi]

The ReadTheDocs project page of the Guidelines tells us that:

Your project(s) "openaire-guidelines-for-literature-repository-managers" 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.