docs: Provide separate conf.py for each manual we want

By default Sphinx wants to build a single manual at once.
For QEMU, this doesn't suit us, because we want to have
separate manuals for "Developer's Guide", "User Manual",
and so on, and we don't want to ship the Developer's Guide
to end-users. However, we don't want to completely duplicate
conf.py for each manual, and we'd like to continue to
support "build all docs in one run" for third-party sites
like readthedocs.org.

Make the top-level conf.py support two usage forms:
 (1) as a common config file which is included by the conf.py
 for each of QEMU's manuals: in this case sphinx-build is run
 multiple times, once per subdirectory.
 (2) as a top level conf file which will result in building all
 the manuals into a single document: in this case sphinx-build is
 run once, on the top-level docs directory.

Provide per-manual conf.py files and top level pages for
our first two manuals:
 * QEMU Developer's Guide (docs/devel)
 * QEMU System Emulation Management and Interoperability Guide
   (docs/interop)

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Message-id: 20190305172139.32662-9-peter.maydell@linaro.org
Message-id: 20190228145624.24885-9-peter.maydell@linaro.org