Connect Antora extensions

[alandefreitas]

The Antora C++ reference extension generates tagfiles, but the tagfile extension expects a separate file. Because they do not coordinate, tagfiles become out of date.

Proposed solution

• Patch both Antora extensions (https://github.com/cppalliance/antora-cpp-reference-extension and https://github.com/cppalliance/antora-cpp-tagfiles-extension) so the reference extension hands its tagfile directly to the tagfile extension.
• Provide configuration toggles in case a site wants to supply an external tagfile.
• Document the integration steps in both repositories.

Requirements

• Tagfiles produced by the reference extension feed the tagfile extension automatically.
• Users can still override the tagfile path.
• Documentation in both projects reflects the integration.