* doc: group major guides in one directory

Moved major guides to one directory without removing files.
Moved images for guides to the guides directory.
This makes it easier to find information
and use cross-linking.

Signed-off-by: Grzegorz Ferenc <Grzegorz.Ferenc@nordicsemi.no>

* Restyled by prettier-markdown

Co-authored-by: Restyled.io <commits@restyled.io>