Create lib subsystem with empty Kconfig menu, subsystem build file.
Updated top-level build and documentation to include subsystem.
This was created to provide an example of how the `lib/` directory
can be connected to the Zephyr build system through `module.yml`,
and so that CI can verify that the extension settings used here
work with these sub-trees.
.
The approach follows the pattern of paralleling the Zephyr tree.
An empty subsystem was implemented rather than a non-empty one
in order to isolate the core changes from any specific library.
This provides a clean reference for users.
.
It was assumed that some users will want to strip out the `lib/`
subsystem separate from existing subsystems, and vice versa.
.
This was verified by:
- visually verifying a clean build of:
`west build -b custom_plank -p always example-application/app/`
- visually verifying the subsystem appeared in the correct
location in a clean build of:
`west build -b custom_plank -p always example-application/app/`
Signed-off-by: Gregory Shue <gregory.shue@legrand.us>
