Switch to MultiMarkdown

[daira]

MultiMarkdown fixes the duplicate reference bug (#893) and also has some nice features like colspan in tables (use $n$ | characters on the right-hand-side of a table entry to make it span $n$ columns).

render.sh added in #953 supports an --mmd flag to use MultiMarkdown. As of that PR you can already use make MARKDOWN_OPTION=--mmd to use this for all Markdown files, provided that the multimarkdown executable is on the PATH.

The reason why I didn't just immediately switch to MultiMarkdown in #953, is that it is not packaged for Linux distributions. It has to be built from source (requiring CMake, for the current version). The build is pretty straightforward; what needs to be done for this issue is to link to the repo and build instructions, and to add it and CMake to the Dockerfile.