Restructure docs

[joshdentremont]

Purpose / why

Per #2387 I took a pass at restructuring the docs and renaming some of the top level categories to make them easier to navigate and more focussed on who each section is for.

What changes were made?

• Removed the release notes page
• Renamed top level navigation
• moved drupal docs into installation and maintenance section
• combined two drupal pages into one and linked between drupal instructions and updating Isle instructions
• added drush commands for config export and import to Isle drupal instructions
• removed Isle-DC from quickstart

Verification

N/A

Interested Parties

• @Islandora/documentation
• @Islandora/committers

Checklist

Pull-request Reviewer

Pull-request reviewer should ensure the following:

• Does this PR link to related issues?
• Does the proposed documentation align with the Islandora Documentation Style Guide?
• Are the changes accurate, useful, free of typos, etc?

Person Merging

The person merging should ensure the following:

• Does mkdocs still build successfully? (This is indicated by TravisCI passing. To test locally, and see warnings, see How To Build Documentation.)
• If pages are renamed or removed, have all internal links to those pages been fixed?
• If pages are added, have they been linked to or placed in the menu?
• Did the PR receive at least one approval from a committer, and all issues raised have been addressed?

[seth-shaw-asu]

👍