[HELP NEEDED] Add all docs

[marten]

It's be really nice if we could get some more hands on this to convert the rest over. I've added all the files here, it's all readable but would look nicer in the new format (see _projects.md for an example). Tag a line below with your name to claim it, so others don't start work on it.

Running and previewing locally is as simple as cd docs; docker-compose up

• classifications
• collection preferences
• collection roles
• collections
• memberships
• organizations
• project preferences
• project roles
• set member subjects
• subject sets
• subject workflow statuses
• subjects
• user groups
• workflows

Review checklist

• First, the most important one: is this PR small enough that you can actually review it? Feel free to just reject a branch if the changes are hard to review due to the length of the diff.
• If there are any migrations, will they the previous version of the app work correctly after they've been run (e.g. the don't remove columns still known about by ActiveRecord).
• If anything changed with regards to the public API, are those changes also documented in the apiary.apib file?
• Are all the changes covered by tests? Think about any possible edge cases that might be left untested.

[lcjohnso]

Closing in favor of #4212