manual: split info on hooks to a separate section/file

Split out the information on hooks to a separate section (and source file).
Not only because the hooks are useful for all infrastructures (and thus
don't really fit specifically in the generic infrastructure section), but
also for clarity when the info on hooks will be expanded in later patches.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Acked-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>