Docs/multiprocessing

[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.

Summary

Part 1 of splitting #3049 into multiple PRs.

• Fixes rendering issues of CustomBlockRNNModule and CustomRNNModule in the documentation.
• Fixes rendering issues of 20-SKLearnModel-examples notebook in the documentation.
• Speeds up the documentation build by utilizing multiple CPU cores.

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[daidahao]

@dennisbader

Looking back, I should have split these issues from the main #3049 and fixed them separately even though they were only minor issues/annoyance at the time. Thank you for this!

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.24%. Comparing base (792bff6) to head (1dc832f).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
darts/models/forecasting/rnn_model.py	81.81%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3094      +/-   ##
==========================================
- Coverage   96.32%   96.24%   -0.08%     
==========================================
  Files         161      161              
  Lines       17346    17350       +4     
==========================================
- Hits        16708    16699       -9     
- Misses        638      651      +13     

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