docs: support other branches about nixvim

[clemenscodes]

Versioned docs would be great. So far only reading the source is viable for anything but latest.

[arunoruto]

It would be beneficial to have the docs of previous Nixvim versions. I am not sure how, or even if, it would be done using the current docs generator, but an example of how it is nicely done is the docs of numba. We can just select what version should be used!

Maybe the docs need to be switched to a different "backend"?

An alternative is to highlight this problem in the readme or even in the docs under FAQ and give a workaround like building the docs locally, as explained here. One can then display the docs using an HTTP server like simple-http-server:

simple-http-server ./result/share/doc/ # the `result` link/folder appears after the build command