👷 ci(docs): add MkDocs Pages workflow (E5-S04)

[konih]

Summary

• Add .github/workflows/docs.yaml: path-filtered push(main) / pull_request / workflow_dispatch for docs/**, mkdocs.yml, README.md, and the workflow itself.
• PR runs mkdocs build --strict (link/nav validation) and uploads the Pages artifact; deploy runs only on main / workflow_dispatch via actions/deploy-pages + actions/upload-pages-artifact.
• All actions SHA-pinned; permissions declare contents: read, pages: write, id-token: write. Does not touch ci.yaml. Pages already enabled (OD-5).

Test plan

• Docs workflow runs on this PR (path filter includes the new workflow file) and build job is green
• Deploy job is skipped on the PR
• After merge to main, site is live at https://platformrelay.github.io/kollect-render/

[sonarqubecloud]

Quality Gate failed

Failed conditions
C Security Rating on New Code (required ≥ A)

See analysis details on SonarQube Cloud

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[konih]

Independent review — APPROVE

Verdict: APPROVE (no P0/P1)

Path-filtered docs workflow; PR builds without deploy; main deploys via Pages; SHA pins. P3: checkout v7 vs ci v4 inconsistency only.