docs: build docs on CI deploy to github-pages on push to main

[tlambert03]

with #38 in, this PR enables publishing docs to a github "environment". This means that docs are served from an uploaded actions artifact (the static site) rather than a checked in branch... and it also means that manual deploys are no longer necessary.

It will build on every PR (so it will be clear if a PR breaks docs) but will only deploy on main. You can download the build artifact from CI as well if you want to take a peak

[codecov-commenter]

Codecov Report

Merging #40 (348313e) into main (8f19c55) will increase coverage by 0.43%.
The diff coverage is n/a.

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@@            Coverage Diff             @@
##             main      #40      +/-   ##
==========================================
+ Coverage   92.52%   92.96%   +0.43%     
==========================================
  Files          28       29       +1     
  Lines         696      725      +29     
==========================================
+ Hits          644      674      +30     
+ Misses         52       51       -1     

see 2 files with indirect coverage changes