Convert the qmp-spec.txt document to restructuredText.
Notable points about the conversion:
 * numbers at the start of section headings are removed, to match
   the style of the rest of the manual
 * cross-references to other sections or documents are hyperlinked
 * various formatting tweaks (notably the examples, which need the
   -> and <- prefixed so the QMP code-block lexer will accept them)
 * English prose fixed in a few places

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20230515162245.3964307-2-peter.maydell@linaro.org>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
[.. code-block:: dumbed down to :: to work around CI failure]