Improvements to spectral indices

[calebrob6]

There are a few improvements we can make to the spectral index transforms in torchgeo:

• Adding a latex rendering of the index in the docs that show the wavelengths of the bands used (with their common names). E.g. the NDRE index uses the NIR and Red Edge bands:
  
• Adding the indices documented in IDB and awesome-spectral-indices
• Documenting common use cases of each index from the associated papers
• Documenting how to interpret index values (e.g. NDVI is between [-1, 1] where -1 is generally unhealthy/no vegetation and 1 is healthy vegetation)

(@MATRIX4284 -- tagging you for visibility as you've been working on the indices)

[adamjstewart]

For math rendering in Sphinx, there are many options: https://www.sphinx-doc.org/en/master/usage/extensions/math.html

PyTorch uses sphinxcontrib.katex, not sure if it has any advantages over the builtin extensions.

[MATRIX4284]

• IDB

i have started working on addingthe other indices.Will update soon