JavaScript update in developer documentation

@nedbat @singingwolfboy @sefk @jinpa 

This is updated to have a general description of JS app requirements, with template example separate.  It also explains the 3 options as discussed in email -- no grading, formative, summative.  Happy to add description of other examples from your JS hackathon if that's reasonable.

Right now, I can't build this on Read the Docs. It's not an issue with these changes -- master won't build either as of a few days ago.  Not sure why.  When I figure it out I'll post a link here.