Docs/fix warnings

[dennisbader]

Checklist before merging this PR:

• Mentioned all issues that this PR fixes or addresses.
• Summarized the updates of this PR under Summary.
• Added an entry under Unreleased in the Changelog.

Fixes #2935.

Summary

• Fixes all warnings when generating the documentation. Also, the documentation can now be generated without having to re-install Darts before every run.
• This is the first step to update our documentation design

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.33%. Comparing base (a0cc926) to head (355b029).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2936      +/-   ##
==========================================
- Coverage   95.40%   95.33%   -0.08%     
==========================================
  Files         146      146              
  Lines       15707    15701       -6     
==========================================
- Hits        14985    14968      -17     
- Misses        722      733      +11     

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