Better conversion of Markdown to PDF

[binkley]

The fast solution to #573 is pandoc, however it relies on the LaTeX toolchain to convert Markdown to PDF, and has limitation.

An alternative approach is other tooling such as https://github.com/phseiff/github-flavored-markdown-to-html.

Example issues:

• No embedded images in the PDF, even when the image files are local to a Markdown file.
• No support for GitHub-style internal links (ie, [[Example page link]]).
• Missing UNICODE characters when LaTeX's default font does not support them (such as 🟢).
  This affects 6 files.

Tech context

(copied from #573)

Note that some UNICODE characters (such as 🟢) do not convert, but the Markdown still converts to PDF.
This affects 6 files.
The underlying issue is with the toolchain used to convert: PDF and Markdown support these just fine.
For PDF it relies on the LaTeX toolchain, and use XeLaTeX to ignore issues with UNICODE.
We need to play with the fonts that LaTeX uses, and find options that both look good, and have the missing characters (the default font is "lmroman10-regular").