Richer documentation: convert to Asciidoc as source of truth

[NWilson]

We will still produce HTML & manpage output, but using the more modern Asciidoc format, which is easier to edit, and supports richer formatting.

Currently our manpage dialect is a little custom already, with comment extensions and postprocessing for the HTML conversion.

Using Asciidoc's engine to produce the manpages should be a little simpler, and its HTML output will be a bit nicer to read.

[NWilson]

Well, it's very much in progress! A large bulk of the transformation is done and merged to the main branch.

I think there will be a tail of cleanup and polishing, but I'm quite pleased so far. It wasn't desperately hard: our man/roff syntax subset is relatively simple (simple enough to have Philip's own in-house Perl script to transform it to HTML). So, building on the back of that and transforming it to Asciidoc wasn't a terrible stretch.