Gallery infrastructure

[jukent]

Related to #126 and #128

Adding our own gallery that uses each viz function instead of relying on geocat-examples so that we can decouple the two packages.

[jukent]

I still want to change installation.rst but that can be outside of this PR.

[jukent]

Going to add links to new examples in docstrings before merging.

[jukent]

../ ../../ and ../../../ in the relative path from fx docstring to example notebook all seem to point to the same broken link: https://geocat-viz--133.org.readthedocs.build/en/133/user_api/examples/add_height_from_pressure_axis.html when we need this with out the user_api folder in place. Is there something about sphinx read the docs that doesn't let us go up another folder? What we want it to point to is https://geocat-viz--133.org.readthedocs.build/en/133/examples/add_height_from_pressure_axis.html

[jukent]

Sorry I changed this from a draft pre-maturely (before I thought to add links between the fx and its example) and I didn't see a way to turn it back. If tests pass this round I think it is actually ready for review.

[jukent]

Yay it's working @kafitzgerald

[kafitzgerald]

Looks good to me!

I know it wasn't the primary objective, but also excited about having usage examples in the viz docs.