Update user 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.