the rules are fixed, and we want to support all of them (or throw a
useful error message). this will also become the base for a generic
renderer system, so let's just list all the rules statically.
this restores mergeJSON to its former glory if…merging json, and
extracts the MD rendering into a new script that will run instead of the
py+nix+xslt pipeline we previously ran to convert options.json to docbook.
this change alone gives a noticable performance boost when building
docs (18s instead of 27s to build optionsDocBook).
no changes to rendered output, except for a single example in the
rsnapshot module that uses hard tabs for indentation instead of spaces.
this probably isn't important.
docbook warnings remain with mergeJSON since the other processing steps
output single files instead of directories. since we'll only keep the
check until 23.11 this is probably also not important to fix.
also contains a few improvements to error reporting in the MD renderers.
