docs/index: Move setup-related topics to configuration section
docs/index: Move setup-related topics to configuration section

- Rename the "Configuration" section to "Setup and Configuration" -
  this section is focused on the ones administrating a nbgrader
  install for others (or themselves).
- Move the "Installation" and "What is nbgrader?" pages to this new
  section.
- The motivation is to keep the "User guide" focused on people using
  nbgrader, something we might send to a instructor or student who
  needs to get started.  The next section starts on things that
  administrators might need to know.  Unlike a lot of software, we
  expect a bigger difference between users and admins here, at least
  for hub-based setups.
- I have not moved the pages, so they are still in the `user_guide/`
  section, not `configuration/`.  While not idea, I consider this less
  bad than moving - for now.  They can be rearranged later.
- Review: does this make sense?  It would be fine to say no.

jupyter/nbgrader,jupyter/nbgrader,jhamrick/nbgrader,jupyter/nbgrader,jupyter/nbgrader,jhamrick/nbgrader,jupyter/nbgrader,jhamrick/nbgrader,jhamrick/nbgrader