Document verified container image lineage permissions

[maximelb]

Summary

• Explain the read-only Artifact Analysis occurrence role for verified Cloud Build image provenance, including the image project scope.
• Document GitHub App Attestations: Read-only for new and existing installations.
• Clarify that unsigned source claims remain asserted.

Validation

• mkdocs build --strict passed.
• Reviewed the diff for private identifiers and credentials; none included.

This is a public documentation PR and is left open for maintainer merge.

[maximelb]

Adversarial self-review of pristine commit ee3ef43: checked role scope, installation upgrade instructions, links, and public-content boundary. Found and fixed one ambiguous placement of the GCP registry-reader bullets in follow-up commit ee3ef43. No open findings. mkdocs build --strict passes. Leaving this public PR for maintainer merge.

[maximelb]

Adversarial self-review from pristine git archive e7e005f: checked the GCP role and image/occurrence project scope, existing App installation permission acceptance, G2 owned/third-party/unknown semantics, links, and public content for internal project names or customer data. mkdocs build --strict passes on the exact head. No open findings. This public PR remains for Maxime to merge.

[maximelb]

Adversarial self-review of exact head f707d9c3 from a pristine git archive extraction, compared with the reviewed prior head and the public setup flow. The final edit resolves an ambiguous permission sentence: registry pull access controls image scanning, while roles/containeranalysis.occurrences.viewer controls Cloud Build provenance verification. The grant example still targets the image/occurrence project and the connected service account. I checked the changed pages for internal project names, tenant identifiers and PII; none are present. git diff --check passes. No open correctness issue. Final exact-head documentation CI is running.