Addresses #177. Sphinx docs haven't been updated in ages.
Updated index.rst and api.rst, including starting to reorganize api's listing for improved readability
Started using the label "readers" for APIs
Reorganized api.rst APIs (readers) into "Readers for Global to USA-national data" vs "Readers for USA regional (sub-national) data"
Fixed many docstring formatting issues, to address sphinx warnings. This resulted in the exposure of docstring blocks that were not being shown (for NWIS and others) due to formatting errors
Fixed intersphinx_mapping error in conf.py
Updated INSTALL to current conda env creation statement
Added sphinx-related packages to conda env, so the development environment includes everything that's needed to update and test the docs
Addresses #177. Sphinx docs haven't been updated in ages.