html_baseurl and READTHEDOCS variables

[ru-fu]

See https://about.readthedocs.com/blog/2024/07/addons-by-default/

The html_baseurl and READTHEDOCS variables will be deprecated. We don't use them in the starter pack, so most likely, we are not affected.
However, several of the packages we install use them (mainly Furo and OpenGraph), so we'll need to verify that these packages include the required changes. If they don't, we'll need to include the code from the blog post in the starter pack.