Improve developer doc

[pllim]

As part of my work at STScI, I will attempt to improve the documentation aimed at developers interested to write their own Ginga plugins.

To-do list from Scipy 2018 (see #679):

• [x] breakout into user and dev manual
• [x] add section of Jupyter widgets to dev doc; link directly to example notebooks on repo for now without rendering
• [x] move FAQ down
• [x] clean up installation instructions

More todo:

• [ ] Write up https://github.com/ejeschke/ginga/pull/639#issuecomment-412305840
• [ ] Ginga callbacks (#85)
• [ ] grc interface and usage (#693 and #681)

[ejeschke]

This is excellent!

[pllim]

I didn't get a chance to do this myself during the sprint, but @hcferguson (and maybe even @gderosa2004) made some edits. @hcferguson plans to open a PR for his edits.

[ejeschke]

See also Issue #85.

[pllim]

Also #693 and #681