Remove arcanist projects from documentation
Summary: Ref T7604. Remove references to "arcanist projects" from documentation.
Test Plan: N/A
Reviewers: Blessed Reviewers, epriestley
Reviewed By: Blessed Reviewers, epriestley
Subscribers: Korvin, epriestley
Maniphest Tasks: T7604
Differential Revision: https://secure.phabricator.com/D13005