Make Doxygen output a .html file.

No .xhtml extension implies no need for a special .htaccess file!
Trivial: style(9) fix. I think.
Check in default Doxygen CSS file.
Harmonise code listing classes with Doxygen's.

Use the same HTML element class names as Doxygen for code listings (e.g. "keywordflow" rather than "control").
Prefer UCam, TESLA styling to Doxygen defaults.

Drop Doxygen CSS that conflicts with UCam or TESLA style.
Highlight some literals.
UCam-ify API documentation.
Don't repeat TESLA's name.