docs/interop: Delete qmp-intro.txt

qmp-intro.txt is quite small and provides very little information
that isn't already in the documentation elsewhere.  Fold the example
command lines into qemu-options.hx, and delete the now-unneeded plain
text document.

While we're touching the qemu-options.hx documentation text,
wordsmith it a little bit and improve the rST formatting.

Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20230515162245.3964307-4-peter.maydell@linaro.org>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>