Make sure docs cache rebuilds + organise tutorials + document dt_mass_modifiers

[TomWagg]

I got carried away and this PR is actually 3 things sorry 😅

Docs caching

@katiebreivik pointed out that rebuilding the docs was super slow. I tracked down a few issues and no rebuilds are super fast locally, only edited files are changed :)

Why it was slow is basically two reasons. 1) The thing where BSEDict is generated dynamically meant that every time it dynamically inserted it then the file was marked as changed and removed from the cache. Now I first make it check whether the settings have actually changed and, if not, leave it in the cache. 2) In interface.rst we were including image directives that were referencing files made at fixed times and that also messed up the cache. Now [at]savefig handles all that.

(...let's not discuss who wrote the four files that were causing the issues 😇 )

Tutorials organisation

I re-organised anything that was a tutorial into a clearer collection so it's easier to navigate. There's also a landing page at tutorials.rst that puts them all together neatly in a grid. Note this will change the URLs of these pages because they are in folders, but should only be an issue if someone has it hardcoded somewhere for some reason.

dt_mass_modifiers docs

I wrote up a docs page for dt_mass_modifiers finally so this will fix #758

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.31%. Comparing base (8772c07) to head (32d7a72).
⚠️ Report is 166 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff              @@
##           develop     #792       +/-   ##
============================================
- Coverage    86.91%   76.31%   -10.60%     
============================================
  Files           40       58       +18     
  Lines        25542    28097     +2555     
  Branches         0      919      +919     
============================================
- Hits         22198    21440      -758     
- Misses        3344     6353     +3009     
- Partials         0      304      +304     

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[TomWagg]

@katiebreivik highlighting that I added my name to the bit that shows up at the bottom of each docs page (so that you can veto if you like, no worries either way! 🙂 )