Improved documentation for search configurations

This reverts commit 7220197a.

Added documentation for `search_tokenizer_separator`.

Mathjax config options added.
`compress_html` config option changed,
Mathjax script loaded by head when `page.mathjax` is truthy.
Documentation and test page added.
See also issue #198.

This commit makes the dark mode preview feel a little more interactive
and it also pulls some duplicated code into one file so that people like
me aren't stumped when they are looking at the wrong page!

The indent is required for the aux links to work.