docs: Assign classes to documentation elements

docs: Assign classes to documentation elements

In CSS the following class names are available:

* keyword     (keywords like "typedef", "struct")
* type        (types like "int", "void*")
* comment     (comments after members of enums or structs)
* directive   (preprocessor directives, #define)
* undisclosed (text saying that the API is not public)

Additionally, kill all of the left-over "programlisting" class
assignments. There are no CSS rules for them.

Signed-off-by: Claudio Bley <cbley@av-test.de>