Update Shpinx documentation

[tcmetzger]

Updated documentation is available at https://pyrte-rrtmgp.readthedocs.io/, including information about the additional low-level and high-level interface features added in Phase 2. This also includes information about the conventions used in the Python frontend. The documentation also includes usage examples (based on the existing usage examples / tests in the repo as much as possible) and provides the basis for building a documentation following the Diataxis framework.

[tcmetzger]

Waiting for #55 to be ready

[tcmetzger]

Updates to readme in #111

[tcmetzger]

ToDos:

• Update user docs for new Python frontend
• Link to (or include) example notebooks
• Update installation instructions to reflect new conda package and dev install (with RTE-RRTMGP as conda dependency instead of local build)
• Update contributing docs about how to make a conda release Update documentation for release (conda) #120