docs: switch to sphinx-multiversion fork

[Tomas2D]

The current sphinx-multiversion library is no longer maintained and has severe flaws, which we were trying to tackle with various hacks.

For instance, to retrieve a current version when the text generation is being run, we had to parse version.json in the parent directory. However, we recently discovered that this approach does not work when multiple documentation sources share the same tag; we cannot detect the current version.

Because the library is relatively small, I've made it a part of our repository and added necessary things.

[coveralls]

coverage: 95.153%. remained the same when pulling b20bf6672ddc67790c981781879c421167440f25 on fix-sphinx-multiversion into 59fe6759934e73b627451b97eed9dbc8ca2e25b4 on main.