Fix lots of docs issues, autogen API, switch theme

[ZedThree]

Also update RTD config to v2 (v1 now doesn't work)

[codecov-commenter]

Codecov Report

Merging #293 (ec5ecc1) into master (212b407) will increase coverage by 0.00%. Report is 6 commits behind head on master. The diff coverage is 100.00%.

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@@           Coverage Diff           @@
##           master     #293   +/-   ##
=======================================
  Coverage   68.94%   68.95%           
=======================================
  Files          15       15           
  Lines        3214     3215    +1     
  Branches      792      792           
=======================================
+ Hits         2216     2217    +1     
  Misses        734      734           
  Partials      264      264           

Files Changed	Coverage Δ
xbout/boutdataarray.py	78.35% <ø> (ø)
xbout/boutdataset.py	76.73% <ø> (ø)
xbout/load.py	69.42% <ø> (ø)
xbout/plotting/animate.py	45.37% <ø> (ø)
xbout/region.py	83.59% <100.00%> (+0.03%)	:arrow_up:

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[ZedThree]

I think the failure is because the RTD theme doesn't support the latest sphinx. I'd like to just switch to using the sphinx_book_theme, but installing that will conflict with #294 so that best go in first

[dschwoerer]

Happy to merge it any way, it certainly improves things :+1:

[ZedThree]

Now also switches to sphinx-book-theme and uses autosummary to auto-generate all the API docs.

[ZedThree]

Actually builds on RTD now!