Minor documentation improvements

Summary:

• Link to "importing a repository" from Config next steps, since it's not

obvious (and the article isn't obviously named).

• Some minor doc tweaks.
• Remove "Roadmap" document since it's super out of date and not very useful.

Test Plan: Regenerated and read documentation.

Reviewers: btrahan, jungejason

Reviewed By: jungejason

CC: aran, jungejason

Maniphest Tasks: T743

Differential Revision: https://secure.phabricator.com/D1384