Add prototype for publishing docs to github-pages

[Mr-Geekman]

Before submitting (must do checklist)

• Did you read the contribution guide?
• Did you update the docs? We use Numpy format for all the methods and classes.
• Did you write any new necessary tests?
• Did you update the CHANGELOG?

Proposed Changes

Add multi-version documentation on GitHub Pages.

Closing issues

[github-actions]

🚀 Deployed on https://deploy-preview-1236--etna-docs.netlify.app

[codecov-commenter]

Codecov Report

Merging #1236 (6f99a6d) into master (3cd565b) will increase coverage by 0.11%.
The diff coverage is n/a.

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@@            Coverage Diff             @@
##           master    #1236      +/-   ##
==========================================
+ Coverage   87.88%   87.99%   +0.11%     
==========================================
  Files         186      186              
  Lines       10621    10621              
==========================================
+ Hits         9334     9346      +12     
+ Misses       1287     1275      -12     

see 3 files with indirect coverage changes

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