doc/user: mention libnetwork and a bit of the filesystem hierarchy

Based on the feedback from Maya Posch in https://mayaposch.wordpress.com/2024/01/20/porting-nymphcast-to-the-haiku-operating-system/
Thanks for noting that the information was missing!

Change-Id: I4a9f709b17cb3a4a771f167b5af0fc83843ec5ad
Reviewed-on: https://review.haiku-os.org/c/haiku/+/7406
Reviewed-by: Niels Sascha Reedijk <niels.reedijk@gmail.com>
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>