Bring old repository instructions and guidance forward to new UI

Summary:
Ref T10923. Fixes T10406. This brings most of the guidance/instructions forward:

• Some remained as instructions.
• Some moved to documentation.

Test Plan: Went through all of the sections and hit the help.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10406, T10923

Differential Revision: https://secure.phabricator.com/D15873