posix: move getpid() to Kconfig.procN and doc MULTI_PROCESS

Create a Kconfig option for POSIX_MULTI_PROCESS Option Group
and mark it as experimental.

Currently, Zephyr has two functions that belong to this
Option Group:
- sleep()
- getpid()

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>