Add formulas to indices docs

[adamjstewart]

See #387

We may need to develop a consistent naming scheme for bands. For example, some indices take in the index of the red-edge band, but many satellites don't have a red-edge band, so we use red instead. The parameter names should be somehow consistent in this scenario.

I haven't yet added common use cases and interpretation to the docs. That can probably be done in a separate PR.

[calebrob6]

The formulas look really small (e.g. https://torchgeo--400.org.readthedocs.build/en/400/api/transforms.html), is it just me?

[adamjstewart]

Looks normal to me on macOS with Chrome:

[calebrob6]

Cool, going to assume it is just me (maybe some weird global mathjax settings I have?)

[adamjstewart]

If other people report the same problem there are other options: https://www.sphinx-doc.org/en/master/usage/extensions/math.html

I chose mathjax because it seemed simpler than imgmath (which requires LaTeX to build image files) and newer than jsmath (seems like an older version of mathjax). There are many third-party options as well (PyTorch uses sphinxcontrib.katex)