📝 Improve pip install zen3geo instructions with extras dependencies

[weiji14]

Made a table of all the possible 'extras' installation options, and what dependencies they come with. That's for the readthedocs index page. The GitHub installation instructions remains minimal, pointing to the detailed table instead for those seeking more.

Preview at https://zen3geo--40.org.readthedocs.build/en/40/index.html

Command	Dependencies
pip install zen3geo	rioxarray, torchdata
pip install zen3geo[raster]	rioxarray, torchdata, xbatcher
pip install zen3geo[spatial]	rioxarray, torchdata, datashader, spatialpandas
pip install zen3geo[vector]	rioxarray, torchdata, pyogrio[geopandas]

References:

• https://jupyterbook.org/en/stable/content/references.html#reference-section-labels