Split README into separate documenation pages

[pgarrison]

Issues addressed

• README is dense and organization is unclear. We want to write to multiple audiences who want to use this for different purposes.
• Docs include paths in tree diagrams that I didn't update when moving to the src/ and subpackages/ structure.
• image_preprocessing usage instructions say --cores .. but .. is just a placeholder (Readme for preprocessing #39)
• Preprocessing docs make dependence on two virtual environments more clear

Reviewing

See new version here

What do you all think about moving the reproducibility and dev documentation out of the main README and instead highlighting the "educational" example(s) @ritvikvasan proposed there?

[ritvikvasan]

Looks much better to me!

[vianamp]

I like this. As mentioned in Slack, replace the "movies" in the figure with single cell images.

[ritvikvasan]

Looks good mostly. I think for the workflow, we should split up the preprocessing steps between punctate and polymorphic models.