doc: updated doc for Zephyr release 1.12.

Updated doc for release 1.12 for :-

Kernel:
Added k_thread_foreach API

Libraries / Subsystems:
subsys/disk: Added support for multiple disk interfaces
subsys/fs: Added support for multiple instances of filesystem
subsys/fs: Added Virtual File system Switch (VFS) support

Signed-off-by: Ramakrishna Pallala <ramakrishna.pallala@intel.com>