docs: Move qemu-option-trace.rst.inc to docs/

docs: Move qemu-option-trace.rst.inc to docs/

The qemu-option-trace.rst.inc file contains a rST documentation
fragment which describes trace options common to qemu-nbd and
qemu-img.  We put this file into interop/, but we'd like to move the
qemu-nbd and qemu-img files into the tools/ manual.  We could move
the .rst.inc file along with them, but we're eventually going to want
to use it for the main QEMU binary options documentation too, and
that will be in system/.  So move qemu-option-trace.rst.inc to the
top-level docs/ directory, where all these files can include it via

   .. include:: ../qemu-option-trace.rst.inc

This does have the slight downside that we now need to explicitly
tell Make which manuals use this file rather than relying on
a wildcard for all .rst.inc in the manual.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 20200217155415.30949-3-peter.maydell@linaro.org