Rename readthedocs file

[fabaff]

Description:

The new configuration was introduced with #17642. This change rename the file to make it more clear that's a configuration file.

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass

[balloob]

Is this format supported?

[rohankapoorcom]

Doesn't look it, per the docs, it must be readthedocs.yml in the root of the project.

However, the regex they are using does allow the filename to have a dot prefix. I tested the regex really fast and it does validate the proposed file name.