use `tcolorbox` instead of `framed` for latex code blocks, works
much better, especially as now when using a background colour;
reduce gap after figures and force footnotes to bottom of page;
move most symbology figures to after some text to lessen chance
of subsection headers being orphaned on their own at end of page:
required a number of rewordings;
use named barcodes in example commands, rather than numbered;
fix "lua-crossrefs" filter to allow parentheses around
references;
various other fixes and rephrasings
some code fiddling on affected files
manual: use new lua filter "lua-crossrefs.lua" hacked from
https://github.com/rnwst/pandoc-lua-crossrefs as replacement for
tablenos which broke with pandoc 2.8.2 (get nicer output anyway);
fix "excode39.svg" data $ -> # to avoid shell processing;
document `ZBarcode_HaveGS1SyntaxEngine()`
