readthedocs: Create .readthedocs.yaml

[tmorrell]

❤️ Thank you for your contribution!

Description

readthedocs isn't currently updating, which means that https://invenio.readthedocs.io/ doesn't have the warning that the documentation there is not maintained.

There will likely need to be a configuration change in readthedocs itself in addition to this PR. I don't have access to readthedocs to do that part.

Checklist

Ticks in all boxes and 🟢 on all GitHub actions status checks are required to merge:

• I'm aware of the code of conduct.
• I've created logical separate commits and followed the commit message format.
• I've added relevant test cases.
• I've added relevant documentation.
• I've marked translation strings.
• I've identified the copyright holder(s) and updated copyright headers for touched files (>15 lines contributions).
• I've NOT included third-party code (copy/pasted source code or new dependencies).
  • If you have added third-party code (copy/pasted or new dependencies), please reach out to an architect on Discord.

Reminder

By using GitHub, you have already agreed to the GitHub’s Terms of Service including that:

1. You license your contribution under the same terms as the current repository’s license.
2. You agree that you have the right to license your contribution under the current repository’s license.

[tmorrell]

The CI is never going to complete, since it's totally outdated. Since this repo is not maintained, I think the goal should be a single update to readthedocs, and then archive the repo. Alternatively we could remove the readthedocs site.