docs/index.rst, docs/index.html.in: Reorder manuals

docs/index.rst, docs/index.html.in: Reorder manuals

Now that qemu-doc.html is no longer present, the ordering of manuals
within the top-level index page looks a bit odd. Reshuffle so that
the manuals the user is most likely to be interested in are at the
top of the list, and the reference material is at the bottom.

Similarly, we reorder the index.rst file used as the base of
the "all manuals in one" documentation for readthedocs.

The new order is:
 * system
 * user
 * tools
 * interop
 * specs
 * QMP reference (if present)
 * Guest agent protocol reference (if present)
 * devel (if present)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20200306171749.10756-7-peter.maydell@linaro.org