[DOCS] Reorganize the documentation

[joshdentremont]

I think we should reorganize the docs to move maintenance instructions out from under the installation heading, make it more clear what each section is for, and combine all the Drupal info into one section.

Right now we have something like this:

• installation
  • release notes (super out of date)
  • manual install
  • playbook
  • adding modules
  • docker
    • isle site template
      • docker install instructions
      • docker maintenace instructions
      • drupal update instructions
• walkthroughs
  • stuff about setting up your new islandora site
• using islandora
  • stuff about setting up searching, metadata, and site customizations
• documentation for developers
  • technical documentation
  • drupal update information

I'd like to reorganize this into something more like this:

• installation and infrastructure maintenance
  • release notes (get rid of this)
  • manual install
  • playbook
  • adding modules (move this in with other drupal instructions
  • docker
    • isle site template
      • docker install instructions
      • docker maintenace instructions
      • drupal update instructions (move this to the drupal sections below)
• using islandora for site admins (combine walkthroughs and using islandora)
  • stuff about setting up your new islandora site
  • stuff about setting up searching, metadata, and site customizations
• technical documentation
  • technical documentation
• using drupal
  • drupal update information (this should be everything we need for maintaining drupal, with or without docker)

If folks think this is a good plan I'd be happy to take a stab at a PR to do it.