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.

Note that most of the SRST fragments are 2-space indented so that the
'info foo' documentation entries appear as a sublist under the 'info'
entry in the top level list.

Again, all we need to do to put the documentation in the Sphinx manual
is a one-line hxtool-doc invocation.

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: 20200228153619.9906-22-peter.maydell@linaro.org