doc: west: clean up manifest import details

Move the content related to handling imports that refer to multiple
files handled to right after the ordered list which defines the order
that manifests are imported. The current placement at the end of this
docs section was poorly chosen.

Create subsections for how projects and extensions are decided in the
final resolved manifest.

The project revisions in the top level example manifest fragment are
not important here. Remove them for clarity.

The top level manifest file need not be named west.yml now; adjust
accordingly.

This is prep work for adding a new subsection for group-filter.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>