Switched from markdown to Djot

Switched from markdown to Djot (See SEP 13: Switch to a more reasonable source markup language) for lightweight document markup. Djot offers many conveniences in terms of consistency and reasonability.

This change required syntactical tweaks be made to virtually all documents, but the pay off in terms of having a thoughtfully considered, simple, robust syntax feels significant.

In Djot raw HTML has to be explicitly declared. This has numerous advantages, again mostly having to do with predictability. No longer is it a guessing game as to whether output will be mangled or needs escaping.

Speed of HTML generation has been improved despite a rough implementation that currently shells out to an external Djot parser (Jotdown) as there is no Djot parser currently written for Python.