Documenting the Python APIs

[ax3l]

PICMI APIs and usage is, besides links to outside docs and examples, not really documented on the same level as the C++ app w/ inputs file usage. Let's change that.

• [x] document the standard PICMI APIs in the WarpX docs: #3291 @ax3l
  • [ ] We furthermore need to document the additional PICMI warpx_ kwargs that we allow and not yet standardize. @dpgrote
• [x] As a follow-up, we should add a section on additional APIs that we already expose in WarpX for field data, particle data, boundary scraping, hooks, etc.: #3310 @roelof-groenewald

@dpgrote @RemiLehe @peterscherpelz @roelof-groenewald et al. help wanted :)

[ax3l]

Unassigned, since I am looking for help with the latter two points :-)