use Mamba to build docs and install graphviz

[zacharyburnett]

we can use mambaforge to build docs in an environment and install graphviz (in case dot or graphviz are needed by any Sphinx plugins in the future)

Checklist

• added entry in CHANGES.rst (either in Bug Fixes or Changes to API)
• updated relevant tests
• updated relevant documentation
• updated relevant milestone(s)
• added relevant label(s)

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (0e37e05) 64.98% compared to head (3a3fe1b) 64.98%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #155   +/-   ##
=======================================
  Coverage   64.98%   64.98%           
=======================================
  Files          99       99           
  Lines        5377     5377           
=======================================
  Hits         3494     3494           
  Misses       1883     1883           

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[WilliamJamieson]

The failures are unrelated, they are due to changes in #149 (the matching JWST PR has not been merged yet).