Refactor doc publication: build in GH Actions, Netlify only for non-main branch previews

[ascheman]

Problem

netlify.toml currently carries a full build command that runs the
doctoolchain (Jekyll + Gradle + asciidoctor + …) on Netlify itself:

command = "jekyll build && cd ../../.. && \
           ./gradlew generateDocs && \
           cp -r ./build/docs/html5/ src/docs/website/_site/documentation"

That couples our doc deployment to Netlify's build environment:

• Netlify's image must keep providing JDK + Ruby + the docToolchain
  toolchain at versions we support; any upstream image change can
  break us with no signal until the next push.
• We have no way to reproduce a Netlify build locally other than by
  pushing to Netlify; debugging is painful.
• Every push triggers the same expensive doc build twice — once in
  GH Actions for the gh-pages.yml flow, once on Netlify.

What we want

• One build, in GH Actions, with full visibility and reproducibility.
• main continues to deploy to GitHub Pages (current behaviour).
• Feature/bugfix branches and pull requests get a Netlify preview
  URL automatically, posted as a PR comment so reviewers can click
  through without checking out.

How

• Strip the [build] command out of netlify.toml. Keep only the
  publish directory + any redirects/functions. Netlify becomes a
  pure hosting target.
• Extend the existing GH Actions docs workflow so that, once it has
  produced the rendered site:
  • If the ref is main → deploy to GitHub Pages (today's path).
  • Otherwise (feature/, bugfix/, pull_request) → upload the
    artifact via a pinned-SHA Netlify deploy action, capture the
    preview URL, post it as a PR comment when applicable.
• Pin the Netlify deploy action to a commit SHA per
  githubactions:S7637.

Acceptance criteria

• netlify.toml no longer contains a [build] command.
• Pushing to main continues to update the GitHub Pages site
  with no behavioural change.
• Pushing to a feature/* or bugfix/* branch publishes a
  Netlify preview at a stable per-branch URL.
• Opening a PR adds a comment with the Netlify preview URL.
• The doc build runs only in GH Actions; Netlify just hosts.