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The Apertis platform document is out of date.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>