Provide a PDF of each documentation component version

[mojavelinux]

Providing a PDF of the documentation makes it easier for visitors to read offline and/or on their reading device. It can also make the documentation easier to browse.

Export each documentation component version as a PDF and provide a link to download it. (If a single PDF is too large, it can be broken up into multiple PDFs, as necessary).

The PDF generation should be handled by an Antora extension that uses one of Asciidoctor's PDF converters. The link should appear next to the expand/collapse icon in the navigation on hover.

[mojavelinux]

In addition to PDF, we should also generate a single (non-chunked) HTML page (which would cover #56). It will likely use a similar mechanism, though I don't want to cause this issue to be held up sorting out the details. Therefore, I think it should be handled as a separate issue.

[mojavelinux]

To be clear, the PDF exporter is currently blocked by the release of Antora 3.

[mojavelinux]

In addition to generating a PDF for a component version, it would be nice if the PDF exporter could also generate a PDF for select pages. That way, we could provide a PDF of the AsciiDoc syntax quick reference as a separate document.

[mojavelinux]

Now this is just waiting on a stable release of Antora Assembler. We anticipate wrapping up that development sometime during the summer (and maybe sooner if stars align).