Convert 'how to use this document' into 'dedication' element.

This makes the content appear in the top 'index.html' element,
and disappear form TOC. I could not find another way - disabling
chunking for the first section would almost work, but would retain
it in TOC.
Do no chunk first sections.

This avoids having sections with almost no content.
Update stylesheet.

In particular, remove most of per-element fonts, make font
large, and update styling for code blocks.