Add a repository import troubleshooting guide

Summary: Fixes T8615. Basic overview of the pipeline, where to look for errors, and how to identify/understand/fix/force your way through.

Test Plan: Read documentation.

Reviewers: btrahan, joshuaspence

Reviewed By: joshuaspence

Subscribers: epriestley

Maniphest Tasks: T8615

Differential Revision: https://secure.phabricator.com/D13376