Documentation: KVM: add separate directories for architecture-specific documentation

ARM already has an arm/ subdirectory, but s390 and x86 do not even though
they have a relatively large number of files specific to them.  Create
new directories in Documentation/virt/kvm for these two architectures
as well.

While at it, group the API documentation and the developer documentation
in the table of contents.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20220322110712.222449-2-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>