docs: Set the Sphinx default highlight language to "guess"

docs: Set the Sphinx default highlight language to "guess"

This should eliminate a whole class of markup warnings, at the cost of
occasionally amusing markup choices; we'll have to see if it works out.

Suggested-by: Markus Heiser <markus.heiser@darmarIT.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>