meson: Document build targets, add 'help' target

Currently important build targets are somewhat hard to discover. This commit
documents important meson build targets in the sgml documentation. But it's
awkward to have to lookup build targets in the docs when hacking, so this also
adds a 'help' target, printing out the same information. To avoid having to
duplicate information in two places, generate both docbook and interactive
docs from a single source.

Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://postgr.es/m/20231108232121.ww542mt6lfo6f26f@awork3.anarazel.de

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