Added a section on the `markdown` package to the mu/ examples.

example/mu/Makefile

example/mu/example.dtx

guide/mu/guide.dtx

test/mu/compare-example/expected-econ-lualatex.pdf

test/mu/compare-example/expected-econ-pdflatex.pdf

test/mu/compare-example/expected-fi-lualatex.pdf

test/mu/compare-example/expected-fi-pdflatex.pdf

test/mu/compare-example/expected-fsps-lualatex.pdf

test/mu/compare-example/expected-fsps-pdflatex.pdf

test/mu/compare-example/expected-fss-lualatex.pdf

test/mu/compare-example/expected-fss-pdflatex.pdf

test/mu/compare-example/expected-law-lualatex.pdf

test/mu/compare-example/expected-law-pdflatex.pdf

test/mu/compare-example/expected-med-lualatex.pdf

test/mu/compare-example/expected-med-pdflatex.pdf

test/mu/compare-example/expected-ped-lualatex.pdf

test/mu/compare-example/expected-ped-pdflatex.pdf

test/mu/compare-example/expected-phil-lualatex.pdf

test/mu/compare-example/expected-phil-pdflatex.pdf

test/mu/compare-example/expected-sci-lualatex.pdf