Add a demo of using the Markdown package in OpTeX

[Witiko]

Closes Continues #215.

Tasks

• Add OpTeX example to examples/ and install it to doc/optex/markdown/examples/optex.tex.
• Add OpTeX module to markdown.dtx and install it to tex/optex/markdown/markdown.opm.
• Add OpTeX to tests/.
• Update CHANGES.md.

[Witiko]

Here is a number of renderers, which I haven't got around to defining yet:

• Must have:
  • Table renderer should typeset a table or at least consume its (variadic) number of arguments and produce no output.
  • Definition lists seem to have no counterparts in OpTeX that a cursory glance over Section 1.4.5 of the documentation would reveal.
• Nice to have:
  • Fancy ordered list renderers
  • Content block renderers should expand to \markdownEscape{#3} (not yet defined in the example document for OpTeX, but see the plain TeX definition in markdown.dtx) if #1 is tex, to a command that renders CSV tables (possibly not available in OpTeX) if #1 is csv and \markdownInput{#3} otherwise.
  • Tickbox renderers should expand to suitable characters from a font, or to an image produced using tikz.opm.
  • Strikethrough renderer
  • Subscript and superscript renderers
  • Note renderer
  • Raw content renderers
  • Line block renderer
  • Hard line break renderer
  • TeX Math renderers

When these renderers have been defined in examples/optex.tex, then make -C examples optex.pdf should produce an example PDF document without error.

@olsak: If you can provide some help with the definitions, it would be much appreciated. Otherwise, I will postpone this PR until a future release.

[Witiko]

Definition lists can be defined and used in OpTeX as follows: http://petr.olsak.net/optex/optex-tricks.html#deflists

[Witiko]

I am going for a minimum viable product that we can ship with the 3.0.0 release. This involves the removal of the \markdownInput macro and the typesetting of the external document example.md from the draft in 95d042d as well as the removal of Lua options for which we don't have renderers implemented in f438f55.

[Witiko]

3a8fa25 must be reverted before the merge.

[Witiko]

cdbf447 drops support for TeX Live 2020 due to OpTeX issue, continuing #269.

[Witiko]

Here is my review of the example document for OpTeX produced by CI in cdbf447: optex-review.pdf. After fixing the example document, this PR should be good to merge.