Fix typo in BNotification documentation

HaikuBook: fix warnings generated by Doxygen 1.18.16.

This version generates some warnings and identified some problems in our
documentation. It is also a bit more vigilant about undocumented elements, so
these are now decorated with a brief description. In the BNotification docs I
used a stray \p to create some whitespace between code and an image. This is
now fixed in the CSS instead.

HaikuBook: Initial documentation for BNotification

The implementation file contained some documentation. This has been moved
to the Haiku Book (and is rewritten in most cases). The documentation gives
some insight on how the notification_server works.

Change-Id: I82bafcf57101d4882bdf07e7f731df9cd8adc861
Reviewed-on: https://review.haiku-os.org/c/haiku/+/2299
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>

API Docs: Add placeholder for Key.h, KeyStore.h and Notification.h