hmp-commands.hx: Add rST documentation fragments

hmp-commands.hx: Add rST documentation fragments

Add the rST versions of the documentation fragments.  Once we've
converted fully from Texinfo to rST we can remove the ETEXI
fragments; for the moment we need both.

Since the only consumer of the hmp-commands hxtool documentation
is the HTML manual, all we need to do for the monitor command
documentation to appear in the Sphinx system manual is add the
one line that invokes the hxtool extension on the .hx file.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20200228153619.9906-21-peter.maydell@linaro.org