Add savefig method to PlotCollection

[aloctavodia]

---

📚 Documentation preview 📚: https://arviz-plots--246.org.readthedocs.build/en/246/

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 72.00000% with 7 lines in your changes missing coverage. Please review.

Project coverage is 69.02%. Comparing base (b57aa39) to head (d1ef50b).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/arviz_plots/backend/bokeh/__init__.py	72.72%	3 Missing ⚠️
src/arviz_plots/backend/plotly/__init__.py	25.00%	3 Missing ⚠️
src/arviz_plots/plot_collection.py	83.33%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #246   +/-   ##
=======================================
  Coverage   69.02%   69.02%           
=======================================
  Files          41       41           
  Lines        4704     4727   +23     
=======================================
+ Hits         3247     3263   +16     
- Misses       1457     1464    +7     

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[OriolAbril]

Can you also add something like

arviz-plots/src/arviz_plots/backend/none/__init__.py

Lines 103 to 110 in b57aa39

	def show(chart):
	"""Show this :term:`chart`.
	Parameters
	----------
	chart : chart_type
	"""
	raise TypeError("'none' backend objects can't be shown.")

to get a more informative message if someone attempts to save a "none" backend object?

It will also make things easier when documenting as I can have a general template with the backend api and render one page per backend substituting the backend only

[OriolAbril]

I'll add some docs and merge