Document API management of repositories and fix some issues with creating URIs via API

Summary:
Ref T10923. Primarily documents the process for creating repositories via the API.

Also fixes a couple of issues with repositoryPHID not being set yet when creating URIs via the API.

Test Plan:

• Followed all documented steps to create a new repository.
• Created and edited some new URIs from the web workflow, too.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10923

Differential Revision: https://secure.phabricator.com/D15870