Rewrite of Platform Support Tiers and Artifacts Documentation

[Sascha Grunert]

Hi folks,

SIG Release is proposing a rewrite of the platform support tiers and
artifacts documentation. The current tiers have vague criteria and
lack measurable requirements.

The PR replaces both with concise, accurate documentation that defines
measurable criteria per tier, adds promotion/demotion processes, and
simplifies the process. The artifact listing reflects what we actually
ship.

No platforms are demoted and no artifacts are added or removed.

PR: https://github.com/kubernetes/sig-release/pull/2974

Feedback welcome on the PR or in reply to this thread.

Thanks,
Sascha
- On behalf of SIG Release