Various updates for develop.md

[totten]

See #527

Note that:

• A lot of the edits are reframing -- there is "Basic Development" (which has fewer requirements / lighter process), and then there is also "Stylesheet Development" and "Editor Development" (which both require additional tools+processes).
• There is no change the wrt to "Publication" for the main extension
• Within "Editor Development", it now discusses both (a) using the prebuilt mosaico (default) and (b) extra steps for patching/publishing mosaico.git.