CI: check the anchors of links in the pull request build #36

The link check of the pull request build (#28) runs lychee --offline without --include-fragments, so only the page part of a link is checked. Antora does not check the fragment of an xref either: renaming a section id so that xref:page.adoc#old-id points nowhere gives neither an Antora error nor a broken link, and the check passes. Raised in the review of #28.

Measured

lychee 0.24.2 over a local build of master (same options as build.yml), with and without --include-fragments:

• 379 more broken links, all Cannot find fragment;
• the check takes 3 min 29 s instead of 54 s.

Of the 379:

• 342 point to the id of the target page itself, e.g. chap-resource-conf#chap-resource-conf. Antora renders the document title as <h1 class="page"> without an id, so the browser just opens the top of the page; harmless for the reader.
• 37 point to a section that is not there (31 OpenAM, 4 OpenDJ, 2 OpenIDM), listed below. Most are <<id>> references to a section of another chapter, which the converter resolves to the current page or to index.html. The two .:chap-jee-agent-config.adoc ones are doc-maven-plugin AntoraMojo: xref:./file.adoc is converted to an invalid .:file.adoc commons#315.

Proposal

1. Run lychee with --include-fragments.
2. In .github/build-baseline/check.sh and in the summary of build.yml, drop a broken fragment that equals the name of the target page (lychee's regular expressions have no back references, so this is done on the JSON report).
3. Report the 37 in the product repositories and add them to .github/build-baseline/broken-links.txt with check.sh --update.