doc: add location for standalone documentation
doc: add location for standalone documentation

It is not always desirable to have documentation mixed with code.
This, as an example, applies to documentation about a certain feature
spanning several headers; or example implementations to aid future
developers.

These changes creates a designated place for such documentation.

Signed-off-by: Thomas Guillem <5f50a84c1fa3bcff146405017f36aec1a10a9e38@gllm.fr>

xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc