[Add forge now] Create user guide
Document user guide on How to add forge now on https://docs.softwareheritage.org/user/
- Add link from feature to user guide
Migrated from T4007 (view on Phabricator)
Edited by Phabricator Migration user
Document user guide on How to add forge now on https://docs.softwareheritage.org/user/
Migrated from T4007 (view on Phabricator)