Documentation versioning

[ggraffieti]

Every new version of Avalanche should have it's documentation, and it could be possible to navigate different versions of the documentation (e.g. documentation for Avalanche 1.2.0 or Avalanche 2.0.4).

• Sphinx does not natively support multi-version documentation.
• Some plugins are available but they don't work with autoapi
• Maybe some ad-hoc script that downloads all the tags from github and compile the documentation for any of them?