CI: check the head meta tags and the robots.txt Sitemap line in the pull request build

[vharseko]

#29 adds <meta name="description">, OpenGraph and Twitter card tags to every page (supplemental-ui/partials/head-meta.hbs, meta-description.hbs) and the Sitemap: line to robots.txt (site.robots in antora-playbook.yml). Nothing checks them automatically: they were verified once in a local build (285 pages, each tag exactly once, og:url missing only on 404.html). A later template edit that drops or duplicates a tag would still build green and deploy.

This was raised in the review of #29 (#29 (review)) and deferred: the pull request check it needs comes with #28 (.github/workflows/build.yml), so the check belongs there as a step rather than in a second workflow.

Proposal

Once #28 and #29 are merged, add a step to .github/workflows/build.yml that fails when:

• build/site/robots.txt has no Sitemap: line, or its host differs from site.url in antora-playbook.yml (Antora writes site.robots verbatim, so today only a comment keeps the two in sync);
• a generated page, other than the redirect pages (http-equiv="refresh"), does not carry exactly one each of <meta name="description", og:title, og:description, og:image, twitter:card;
• og:url is missing on any page but 404.html.

The review suggested a starting point for the tag loop:

grep -qx 'Sitemap: https://doc.openidentityplatform.org/sitemap.xml' build/site/robots.txt
fail=0
while IFS= read -r f; do
  grep -q 'http-equiv="refresh"' "$f" && continue
  for tag in '<meta name="description"' '<meta property="og:title"' '<meta property="og:description"' '<meta property="og:image"' '<meta name="twitter:card"'; do
    n=$(grep -cF "$tag" "$f" || true)
    [ "$n" -eq 1 ] || { echo "$f: $n x $tag"; fail=1; }
  done
done < <(find build/site -name '*.html')
exit $fail

The API docs copied into build/site by npm run copyApiDocs must be left out, as the link check of #28 does.

Done when

Removing any of the tags from head-meta.hbs, or the Sitemap: line from the playbook, turns the Build check red.