To document SHM API use examples

[iartemov-ledger]

The SHM API family is already quite well described (https://outpost-sentry.readthedocs.io/en/latest/uapi/syscalls/get_shm_handle.html etc.)
What is missed is:

• some examples of sequencing of actions/calls for real SHM use cases, starting shared memory node(s) definition in corresponding .dts and finishing by its use in application
• some clarifications including:
  • the necessity to set credentials for the owner task itself
  • when the permission transfer is employed
  • the fact that get_shm_handle() can fail on user side if the owner has not yet set corresponding permissions (and the ways to workaround this)

After, should we elaborate on this or sooner promote a higher-level Posix-like SHM API: https://man7.org/linux/man-pages/man7/shm_overview.7.html ?