Route publication contributions through public pull requests

[Brandon Miller (brandom-msft)]

Summary

• make this public repository the sole venue for changes intended for publication
• require sample changes to use same-repository branches because the required trusted gate intentionally rejects fork pull requests
• state that required checks must pass and a maintainer initiates every merge
• remove private-staging, promotion, transition taxonomy, and automated handoff guidance

Scope

This PR changes only README.md and CONTRIBUTING.md. It does not change workflows, rulesets, CODEOWNERS, samples, notebooks, or repository settings.

Activation gate

Keep this PR in Draft until:

1. contributor wording is approved by the designated AI Platform Docs documentation owner;
2. the public trusted validation lane is restored after force-full sync commit 1a7f828fd60e6b0f14549612cf25e7bf2c906931 removed it;
3. P5.2 disables and drains private sync workflow 233646160, keeps public mirror workflow 289478915 disabled, and revokes the legacy sync app's public write/merge/bypass capability.

At activation, merge this public guidance first through the normal human-triggered public merge path, then merge the coordinated private guidance in microsoft-foundry/foundry-samples-pr#895.

Tracking

• ADO 5502654
• ADO 5449716

Validation

• git diff --check
• final changed-file review limited to README.md and CONTRIBUTING.md
• normal pull-request checks, including required trusted, after restoration of the deleted validation lane

[Brandon Miller (brandom-msft)]

Sheri Gilley (@sdgilley) This is ready for AI Platform Docs owner review. The focused diff changes only README.md and CONTRIBUTING.md; P5.2 writer shutdown and public-only routing activation remain blocked on your explicit approval.