Documentation: irqdomain: Update it

The irqdomain documentaion became obsolete over time. Update and extend
it a bit with respect to the current code and HW.

Most notably the doubled documentation of irq_domain (from .rst and .h)
was unified and let only in .rst. A reference link was added to .h.

Furthermore:
 * Add some 'struct' keywords, so that the respective structs are
   hyperlinked
 * :c:member: use where appropriate to mark a member of a struct
 * Rephrase some wording to improve readability/understanding

Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250319092951.37667-57-jirislaby@kernel.org