Robust.Cdn 2.0: multi-fork, built-in server manifest

[PJB3005]

Robust.Cdn 2.0 has just been released and brings with it major changes:

• It can now cleanly service multiple independent forks at once.
• It can now directly manage server builds, and makes the publishing workflow infinitely simpler.

The old publishing workflow has been removed in the upstream repository as part of #30009 and #30018. You can revert these PRs on your fork if you want to avoid having to migrate to the new publishing system for the time being.

The documentation has been updated, and includes a guide to migrate for people using a previous version of Robust.Cdn.