[docs] Add section about different ways of dependency management for coffea-casa AF

[oshadura]

• Add docs about suggested dependency management for users who want quickly/iteratively prototyping analysis on AF (without possibility of pushing in git repo and/or without pip and conda)
• Add suggested dependency management for users who want quickly/iteratively prototyping analysis on AF (with possibility of pushing in git repo and/or with pip and conda)
• Check and document how we can set PYTHONPATH variable at startup: Global PYTHONPATH Variable #137

Suggested by @Andrew42 and @aperloff