Update a couple of links in "Feature Requests" documentation

Summary:
These are a little out of date:

• Link to Starmap since it explicitly exists now.
• Link to "Planning" instead of the old task.
• Link to "Prioritization" instead of telling anyone to build stuff themselves.

Test Plan: Read documentation.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D14328