Change docs to use "furo" theme

[jwodder]

I don't like Alabaster's lack of spacing between consecutive classes, so I thought I'd suggest switching to the latest hotness, furo.

[codecov]

Codecov Report

Merging #790 (78bda98) into master (8313173) will decrease coverage by 0.33%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #790      +/-   ##
==========================================
- Coverage   85.25%   84.91%   -0.34%     
==========================================
  Files          59       59              
  Lines        6041     6041              
==========================================
- Hits         5150     5130      -20     
- Misses        891      911      +20     

Flag	Coverage Δ
unittests	84.91% <ø> (-0.34%)	⬇️

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Impacted Files	Coverage Δ
dandi/conftest.py	38.46% <0.00%> (-61.54%)	⬇️
dandi/support/generatorify.py	0.00% <0.00%> (-16.10%)	⬇️
dandi/download.py	81.69% <0.00%> (-0.55%)	⬇️
dandi/dandiapi.py	92.46% <0.00%> (-0.20%)	⬇️
dandi/_version.py	45.96% <0.00%> (+1.75%)	⬆️

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[yarikoptic]

looks good to me

[yarikoptic]

ok, let's proceed with docs tune up PRs. Thank you @jwodder