Add the Antora xref text extension for spec repos

[Bill Traynor (wmat)]

Adds antora-extensions/xref_text_extension.js, so specification repositories can use it in the Antora builds they run themselves.

Why

A cross-page reference written without link text:

<<chapter.adoc#some-id>>        xref:chapter.adoc#some-id[]

resolves to the right page in Antora, but the link text comes out as the raw target (chapter.adoc#some-id). Specs single-sourced for both the PDF and the site write their cross-chapter links this way, because the PDF build generates the text itself ("Section 2.3", "Table 5", "IRR4"). The extension supplies that text on the site, using the target's reftext or its section or table title.

The same extension already runs on the central dev site (riscv-admin/antora-dev.riscv.org#47). The central playbooks register their own copies, since a site build cannot load code from a content source. It lives here so that the builds a spec repo runs itself get the same treatment:

• the local preview (npm run preview),
• the pull-request preview from validate-content-source.yml,
• the repo's GitHub Pages site.

Effect on repos consuming this submodule

None, unless a repo opts in. The file is inert until a repo registers it in its own antora-playbook.yml:

• The common Antora component is unchanged: Antora publishes antora.yml and modules/ only, so a new top-level directory is invisible to the site build.
• PDF builds are unchanged: they read global-config.adoc, themes/, fonts/ and images/ by path, and no existing file is touched.
• Submodules are a single gitlink to a consuming repo's linters, so repo checks don't see it.

Dependabot will propose the usual submodule bump in consuming repos; merging it changes nothing by itself.

One ordering rule, in the README: a repo should register the extension only once its submodule points at a commit containing it, otherwise Antora fails with Cannot find module. Bump the submodule in the same PR as the registration. This is caught by a repo's own PR check, never on a published site.

Testing

With this branch checked out as the docs-resources submodule of riscv/riscv-high-assurance-cryptography (migrated to the current template), and registered in its antora-playbook.yml:

• the component builds with 0 errors and the extension filled the link text for all 175 cross-chapter links, none left as a raw target;
• scripts/gen-pages-playbook.js carries the registration into the generated GitHub Pages playbook unchanged;
• the file is byte-identical to the copy merged in antora-dev.

🤖 Generated with Claude Code

[Kersten Richter (kersten1)]

LGTM