Enable versioned documentation

[ndgrigorian]

This PR adds a multiversion option to the gen_docs script, and implements it in the documentation generation workflow, to add a sidebar for documentation versions

• Have you provided a meaningful PR description?
• Have you added a test, reproducer or referred to an issue with a reproducer?
• Have you tested your changes locally for CPU and GPU devices?
• Have you made sure that new changes do not introduce compiler warnings?
• Have you checked performance impact of proposed changes?
• Have you added documentation for your changes, if necessary?
• Have you added your changes to the changelog?
• If this PR is a work in progress, are you opening the PR as a draft?

[coveralls]

coverage: 86.248%. remained the same — enable-versioned-documentation into master

[github-actions]

View rendered docs @ https://intelpython.github.io/dpctl/pulls/2276/index.html

[github-actions]

Array API standard conformance tests for dpctl=0.22.0rc1=py310h93fe807_4 ran successfully.
Passed: 1247
Failed: 44
Skipped: 89

[github-actions]

Array API standard conformance tests for dpctl=0.22.0rc1=py310h93fe807_5 ran successfully.
Passed: 1247
Failed: 44
Skipped: 89

[github-actions]

Array API standard conformance tests for dpctl=0.22.0rc1=py310h93fe807_5 ran successfully.
Passed: 1246
Failed: 45
Skipped: 89

[github-actions]

Array API standard conformance tests for dpctl=0.22.0rc1=py310h93fe807_5 ran successfully.
Passed: 1245
Failed: 46
Skipped: 89

[github-actions]

Array API standard conformance tests for dpctl=0.22.0rc1=py310h93fe807_6 ran successfully.
Passed: 1248
Failed: 43
Skipped: 89

[ndgrigorian]

@antonwolfy @vlad-perevezentsev
I've added to gh-pages both 0.21.0 and 0.21.1 docs. I also have docs prepared for 0.20.0 through 0.20.2, but will save to update gh-pages later on

given that this is now working, with other pages, this is ready for review

[github-actions]

Array API standard conformance tests for dpctl=0.22.0rc1=py310h93fe807_7 ran successfully.
Passed: 1248
Failed: 43
Skipped: 89