docs: Move pr-manager.rst into the system manual

Move the pr-manager documentation into the system manual.
Some of it (the documentation of the pr-manager-helper tool)
should be in tools, but we will split it up after moving it.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>

docs/system/index.rst

@@ -31,6 +31,7 @@ Contents:
    managed-startup
    cpu-hotplug
    virtio-pmem
+   pr-manager
    targets
    security
    deprecated