docs: Split out sections for the manpage into .rst.inc files

Sphinx doesn't have very good facilities for marking chunks
of documentation as "put this in the manpage only". So instead
we move the parts we want to put into both the HTML manuals
and the manpage into their own .rst.inc files, which we can
include from both the main manual rst files and a new toplevel
rst file that will be the skeleton of the qemu.1 manpage.

In this commit, just split out the parts of the documentation
that go in the manpage.

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: 20200228153619.9906-29-peter.maydell@linaro.org