doc(syntax_highlight_in_latex.md): explain how to use minted and Pygments
doc(README.md): add syntax_highlight_in_latex

[skip ci]
chore(frontends/lean/parser): minor cleanup
feat(frontends/lean): variables/parameters and check commands have access to all section variables/parameters, closes #231
feat(bin/linja): add --keep-going N option

Related issue: #234