feat: serve TechDocs from GitHub Pages via custom publisher

[Copilot]

Backstage was building and rendering TechDocs locally while the same docs were already pre-built and hosted on GitHub Pages. This switches Backstage to read directly from GitHub Pages, eliminating redundant local builds while preserving search indexing.

New plugin: techdocs-backend-module-ghpages

Implements PublisherBase from @backstage/plugin-techdocs-node as a GitHub Pages proxy:

• docsRouter() — maps /namespace/kind/name/… → baseUrl/name/…, streams response
• fetchTechDocsMetadata() — fetches techdocs_metadata.json from baseUrl/name/
• hasDocsBeenGenerated() — HEAD-checks baseUrl/name/index.html
• publish() — intentional no-op; builds happen in repo CI

Registered via techdocsPublisherExtensionPoint under the custom key 'githubPages'.

Config change

# app-config.yaml
techdocs:
  builder: "external"
  publisher:
    type: "githubPages"
    githubPages:
      baseUrl: "https://radiorabe.github.io"

Search indexing continues to work: the TechDocs collator reads through the same publisher.

What each repo needs to do

Documented in docs/techdocs.md and plugins/techdocs-backend-module-ghpages/README.md. In short: build with techdocs-cli generate (which produces techdocs_metadata.json alongside the HTML) and deploy the entire ./site output to gh-pages. The metadata file must be present at the root of the GitHub Pages site or Backstage cannot display the docs.

# .github/workflows/techdocs.yml (minimal)
- run: |
    npm install -g @techdocs/cli
    pip install "mkdocs-techdocs-core==1.*"
    techdocs-cli generate --no-docker
- uses: peaceiris/actions-gh-pages@v4
  with:
    github_token: ${{ secrets.GITHUB_TOKEN }}
    publish_dir: ./site        # includes techdocs_metadata.json

Entity metadata.name must match the GitHub repository name (lower-case) since the publisher maps entity name directly to the GitHub Pages path segment.

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