Write concise user documentation

[piotrszul]

Write a concise user manual using sphinx and make is available at readthedocs. The audience should be primarily bioinformaticians. The scope should coverL

• installing variant spark
• running importance analysis with command line an python APIs
• references for available functions and options
• running on cluster and clouds (AWS, Databricks)
• a simple developer guide (building testing).

The proposed outline is here (CSIRO internal only): https://docs.google.com/document/d/1kIZ69VoDTdQhhC0eLBYM_w3v9bG77ZB2lbOYfdXKaWU/edit

The preview of the version committed to the feature branch [ _i76_docs ] is at: https://variantspark.readthedocs.io/en/i76_docs/