Add legend method to PlotCollection and improve docs

[OriolAbril]

I have started working on a legend method for PlotCollection, so far it works with matplotlib, but still quite unclear what the ideal behaviour should be.

In parallel, I plan to extend the docs of mostly everything.

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📚 Documentation preview 📚: https://arviz-plots--32.org.readthedocs.build/en/32/

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 37.20930% with 81 lines in your changes are missing coverage. Please review.

Project coverage is 61.29%. Comparing base (0651970) to head (171225b).

Files	Patch %	Lines
src/arviz_plots/backend/bokeh/legend.py	15.62%	27 Missing :warning:
src/arviz_plots/plot_collection.py	14.28%	18 Missing :warning:
src/arviz_plots/backend/__init__.py	53.84%	12 Missing :warning:
src/arviz_plots/backend/matplotlib/legend.py	21.42%	11 Missing :warning:
src/arviz_plots/backend/bokeh/__init__.py	37.50%	10 Missing :warning:
src/arviz_plots/visuals/__init__.py	84.61%	2 Missing :warning:
src/arviz_plots/plots/distplot.py	75.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #32 +/- ## ========================================== - Coverage 65.65% 61.29% -4.36% ========================================== Files 11 14 +3 Lines 693 801 +108 ========================================== + Hits 455 491 +36 - Misses 238 310 +72 ```

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[OriolAbril]

Should be ready to merge, we can continue iterating on the legend method and on documentation.