developer_guides: debugability: shell: add usage documentation

[kv2019i]

Add basic introduction and usage documentation on how to use Zephyr shell feature with SOF.

[kv2019i]

Keeping as draft until the implementaion PRs get merged.

[kv2019i]

Marking ready for review as the shell backend has been merged in Zephyr and the code has been pulled into SOF main.

[kv2019i]

v2:

• missing "-p" in the examples, bug found by @jsarha