doc: west: move 'manifest imports' to bottom of manifest.rst

This diff is deceptively complicated. No text is being added or
removed; the only thing that's being done is that the entire 'Manifest
Imports' section is being moved down below the groups and submodules
sections.

This is prep work for a later commit which will document revised
semantics for mixing 'import' with 'group-filter'. This will make
'group-filter' values compose across imports, so it will be useful to
have documentation for group and group-filter before documentation for
imports. That will let us document the way they work together within
the 'Manifest Imports' section without using concepts that haven't yet
been defined.

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