Update the Apertis platform document

The Apertis platform document is out of date. Update it to reflect some changes to the Apertis project that have been true for a while:

• We use GitLab rather than straight git repositories
• All code is stored in GitLab (rather than as packages in OBS)
• We use GitLab for code review rather than Phabricator
• Automation has been moved from Jenkins to GitLab CI/CD
• We don't interact directly with OBS much any more
• We use OSTree for OTA updates

Also, remove the comparison with Yocto as this is neither overly balanced, accurate nor useful.

Signed-off-by: Martyn Welch martyn.welch@collabora.com