feat: DocBook class overhaul

[Omikhleia]

WIP = Runny code for #1338 ^^

This ugly baby "decently" processes most of the native Markdown chapter from markdown.sile, converted to DocBook with Pandoc, i.e. it should cover a decent subset of DocBook as generated via Pandoc).

It processed less than decently a big technical documentation in DocBook, but at least it doesn't crash.

[Omikhleia]

This old PR never came to fruition as-is and now has conflicts.
Due to inactivity, and as part of backlog cleaning, I would have been tempted to close it, but the topic was interesting, and the reasons why it didn't occur in this case are quite clear.

So I'm adding the "pending closure" as a label - As a last chance for contributors to eventually say their word.
After some period of time, this inactive PR will be closed without further notice.

But I'd like to also question then:

• The removal of the non-working and unmaintained docbook class from the core distribution
• The replacement of all mention to it and all fake and unmaintained examples on "XML processing" in the User Manual
• The eventual death of the xmltricks package (it's a shame)
•  New approaches to XML processing (SILE and XML - A Return on Experience #2111)

[Omikhleia]

The removal of the non-working and unmaintained docbook class from the core distribution

Another solution would be for this package/class mess to be extracted to a separated (3rd-party) module (distributed on luarocks etc.), while being under the "umbrella" of the SILE organization. (The main idea here implies that at least some people are interested in fostering it further, without impediments).

Yet, I wonder if it is worth the effort, as DocBook > Pandoc > Markdown/Djot > SILE > PDF is still a possible workflow with better results so far (Or So I Claim).