sphinx theme from conda-forge

[brian-rose]

Testing whether we can build with the theme from conda-forge instead of pip

[github-actions]

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[brian-rose]

Had to remove the obsolete version pins in the cookbook actions, ProjectPythia/cookbook-actions#108

[brian-rose]

This is working! We are now installing the sphinx theme from conda-forge.

[brian-rose]

The goal of this PR is to simplify the environment file in the Cookbook template as much as possible. We want to let conda / mamba handle all the dependencies.

[erogluorhan]

Please hold this for now as there is a dependency mismatch in Conda version that showed up in. the portal tests I ran.

[erogluorhan]

This can be closed now as PR #163 handled this and got merged.