Update documentation for GitLab usage

Now that the migration to GitLab is done, the development documentation needs to be updated to remove any mention of Phabricator.

Phabricator is currently mentioned in:

• Docs setup.py
• Devel / Getting started / Developer setup
• Devel / Getting started / Run your own Software Heritage
• Devel / Contributing / Submitting patches (#4722 (closed))
• Devel / Contributing / Code review (!333 (merged))
• Devel / Contributing / Git style guide (!332 (merged))
• Devel / Tutorials / Add new package (!329 (merged))
• Sysadmin / Deployment / Debian packaging
• Sysadmin / Deployment / Jenkins
• Sysadmin / Network architecture / DNS servers
• Sysadmin / Puppet / How to manage Third-Party modules
• Sysadmin / User management / Onboarding checklist
• Sysadmin / User management / Outboarding checklist

Once this is done:

• Remove phabricator.rst entirely