I'm maintaining the fork of the multiversion extension the theme is using under my GitHub account.

This PR moves the contents of the sphinx-multiversion repository to maintain both projects as a monorepo.

Main benefits:

• Collaboration: Other ScyllaDB contributors will be able to update the code, not only me.
• Simplicity: Makes more convenient to test the multiversion extension with a real project while developing new features.
• Reusability: We can reuse some tools for both projects (linters, workflows, deployment process, ...).