[REQ] Publish previous versions of Sphinx documentation

[shi-eric]

Description

Users want a convenient way to browse the docs for older Warp versions. Something like sphinx-multiversion should make this possible (https://sphinx-contrib.github.io/multiversion/main/index.html).

Some quick notes:

• This won't work out of the box. I've tried it before.
• At some point, I think we required that the Warp library be compiled in order to build the docs. We should revisit this restriction.
• Similar to the above point, I suspect multiversion won't like the import warp as wp in conf.py.
• Ideally when running build_docs.py locally, the default behavior is to build for only the current version. The multi-version build should be opt-in.