Docs/general improvements

[dennisbader]

Summary

General documentation improvements:

• added model links to README.md and grouped them into model categories
• added model links to covariates user guide
• reorganized the forecasting model API section to better find models and see their model category
• added the DTW (dynamic time warping) to the documentation and improvement it
• some necessary updates to DTW module from changes over the years

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Codecov Report

Patch coverage: 93.75% and project coverage change: -0.02 ⚠️

Comparison is base (933316b) 94.00% compared to head (cea95c2) 93.98%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1904      +/-   ##
==========================================
- Coverage   94.00%   93.98%   -0.02%     
==========================================
  Files         126      126              
  Lines       11905    11895      -10     
==========================================
- Hits        11191    11180      -11     
- Misses        714      715       +1     

Impacted Files	Coverage Δ
darts/dataprocessing/dtw/__init__.py	100.00% <ø> (ø)
darts/models/forecasting/tide_model.py	97.22% <ø> (ø)
...arts/models/forecasting/torch_forecasting_model.py	90.46% <ø> (-0.04%)	⬇️
darts/dataprocessing/dtw/window.py	85.62% <87.50%> (-0.46%)	⬇️
darts/dataprocessing/dtw/dtw.py	94.28% <100.00%> (+0.08%)	⬆️

... and 5 files with indirect coverage changes

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[madtoinou]

I really like how you changed the tables summarizing all the available models, much clearer in my opinion!

Found a typo and a little inconsistency.