Use Playground v3 reusable workflows for docs previews

[adamziel]

Summary

This swaps the custom docs preview publisher for the Playground PR Preview v3 reusable workflows.

The docs-specific build script still creates the DevHub snapshot, but the generic artifact publishing, release asset management, URL substitution, and PR comment handling now live in WordPress/action-wp-playground-pr-preview instead of this repository.

Changes

• Use preview-build.yml to run the docs snapshot build and stage docs-playground-snapshot.zip as a named Playground preview artifact.
• Use preview-publish.yml with a custom Blueprint that unzips the generated snapshot into /wordpress and restores siteurl/home at runtime.
• Remove the bespoke publish.mjs implementation.
• Keep the publish workflow in comment mode so it does not rewrite the PR description.

Validation

• git diff --check
• node --check .github/scripts/docs-playground-preview/build.mjs
• Parsed both workflow files as YAML and parsed the publish Blueprint as JSON.
• uvx zizmor .github/workflows/docs-playground-preview-build.yml .github/workflows/docs-playground-preview-publish.yml

Note: I did not rerun the full 30-minute snapshot build locally. The existing snapshot-shape failure in the parent PR may still need a separate fix.