Mkdocs config

[BalzaniEdoardo]

• Created a gallery_config.py file to facilitate sorting tutorials in the documentation index based on the alphabetical order of the tutorial script names.
• Renamed the tutorials so that each file begins with "plot_N_...", where 'N' represents a two-digit number indicating the display order in the documentation index.
• Configured the gallery in mkdocs.yaml to execute the configuration script during the build process.

[billbrod]

I think we need some more explanation in the configure script.

• Is it possible to sort the tutorials using something other than alphabet sort? This is what I do for plenoptic as well, but it seems a bit odd to expose it so much to the user. Something like a variable or custom function that determines the order (like sorted accepts) which would go in the configuration somewhere.
• Is it possible to modify the name of the scripts that get built for tutorials? We probably don't want them all to start with plot_. Is there a reason not to just use every .py file in examples/?
• Probably not necessary yet, but is it possible to break the tutorials into groups? So we have one set for "Basis objects", one for "Building design matrices", etc.

[billbrod]

Still want a bit more details in that config, questions about reset argv, and the bigger picture questions I posted in my last review (pasted below). Not all of those need to be addressed in this PR, but I'd like a bit of clarity on them, maybe create relevant issues.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6f210b4) 95.02% compared to head (9ce0b91) 95.02%.

Additional details and impacted files

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  Files          10       10           
  Lines         864      864           
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  Hits          821      821           
  Misses         43       43           

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[billbrod]

Alright, this looks good to me!

Answering my comments from earlier:

• conf['within_subsection_order'] gets passed to sorted and called on the filenames (within a subsection, as that name implies; we currently only have every tutorial in one subsection), so anything that works with sorted will work there.
• we now execute every .py file within docs/examples/. The default behavior is to include (but not execute) every .py file, and only execute those that start with plot_ (I'm not sure why you would want to include but not execute a file -- I guess if it took too long?). See sphinx-gallery docs for more details on how to control that behavior (you can also exclude things entirely).
• Reorganize Tutorials hierarchically #76 will handle the organization of tutorials.