docs/system: move x86 CPU configuration to a separate document

docs/system: move x86 CPU configuration to a separate document

Currently, cpu-models-x86.rst.inc is included in target-i386.rst directly.
To make the toctree more homogeneous when adding more documentation,
include it through a first-class .rst file.

Together with the previous changes to the man page skeletons, this also
frees "===" for the headings, so that cpu-models-x86.rst.inc need not
assume anything about the headings used by target-i386.rst.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>