Establish procedure for including non-graphical figures.

For passages that are too complicated for Markdown, a facility has been
introduced for injecting external TeX code when generating PDF, and HTML
code when generating HTML. The three sample figures that demonstrate
this may be removed again once the sample code (which is after all a
real document) has moved to a different project for maintenance.
Adjust HTML output to match DCC website style more closely.

Specifically, convert citations to footnotes, wrap footnote markers in
brackets, and insert 'Back to top' links before section headings.