Allowing the gitea administrator to configure sanitization policy allows
them to couple external renders and custom templates to support more
markup. In particular, the `pandoc` renderer allows generating KaTeX
annotations, wrapping them in `<span>` elements with class `math` and
either `inline` or `display` (depending on whether or not inline or
block mode was requested).

This iteration gives the administrator whitelisting powers; carefully
crafted regexes will thus let through only the desired attributes
necessary to support their custom markup.

Resolves: go-gitea#9054

Signed-off-by: Alexander Scheel <[email protected]>

 - Adds basic documentation to app.ini.sample,
 - Adds an example to the Configuration Cheat Sheet, and
 - Adds extended information to External Renderers section.

Signed-off-by: Alexander Scheel <[email protected]>

Signed-off-by: Alexander Scheel <[email protected]>

These were left over from their initial names. Make them singular to
conform with the current expectations.

Signed-off-by: Alexander Scheel <[email protected]>