fix: Fix documentation build with extra dependencies

[jules-ch]

Closes #565

Proposed Changes

• Upgrade Sphinx versions & sphinx_rtd_theme
• Install optional dependencies for documentation build
• Update .readthedocs.yml to V2 format
• Update conf.py

Checklist

• CHANGELOG.md updated
• Rebased/mergeable
• A test has been added if appropriate
• pytest tests completes successfully
• Commit messages are conventional
• Sign CLA (if not already signed)

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: -0.02 ⚠️

Comparison is base (a4c5746) 90.30% compared to head (fd07c38) 90.29%.

❗ Current head fd07c38 differs from pull request most recent head 0660d8b. Consider uploading reports for the commit 0660d8b to get more accurate results

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #566      +/-   ##
==========================================
- Coverage   90.30%   90.29%   -0.02%     
==========================================
  Files          39       39              
  Lines        3456     3452       -4     
==========================================
- Hits         3121     3117       -4     
  Misses        335      335              

see 5 files with indirect coverage changes

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[bednar]

Hi @jules-ch,

thanks for your PR 👍.

Can you please rebase the sources with master branch? After that we are be able to merge this PR.

Thanks

[jules-ch]

Alright I'll rebase this.

[jules-ch]

Rebased on master

[bednar]

@jules-ch thanks again for your PR 👍