doc: move runner documentation out of west-apis.rst

Now that the runners package no longer depends on west, we can move
the documentation for it from the orphan west-apis.rst page to the
build-flash-debug.rst page that documents the west extension commands
which use that package.

(This is a more logical place for this information, but it was
previously not possible to put it there since we must be able to build
the documentation on a system without west installed, excepting
content in west-apis.rst. Removing the dependency means the runners
automodule directive can safely appear in build-flash-debug.rst.)

Signed-off-by: Marti Bolivar <marti.bolivar@nordicsemi.no>