Add versioning support to documentation website

[jeluard]

• do we wanna look into adding versioning, or at least show the lasted version here?
  

Originally posted by @nflaig in #6533 (comment)

[philknows]

I do agree with @nflaig comment here about current + a few verions back: #6559 (comment)

If we need a specific number, I doubt people run more than 5 minor versions back.

[jeluard]

Summarizing needs:

• unstable, referring to unstable branch
• stable, referring to stable branch
• some fixed number of versions back

Ideally this should be automated. So any changes in docs in the unstable branch would trigger the update of the documentation website.
Older versions (beyond the specific number we would agree on) should be automatically deleted. Each new version is a complete duplicated folder.
Somehow this process should hook with release process, but not interfere with the unstable docs.