manifest: Update zephyr module

Update zephyr to include 3 pending upstream PRs.

Signed-off-by: Aksel Skauge Mellbye <aksel.mellbye@silabs.com>

doc: Add introductory information

Add introductory information to the README, and move Linux
getting started details to a dedicated document.

Signed-off-by: Aksel Skauge Mellbye <aksel.mellbye@silabs.com>