docs: vm: index.rst: add an orphan doc to the building system

The new free_page_reporting.rst file is not listed at the index.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/146432ae6965a2bb62c929a6b62f9d4010986622.1586881715.git.mchehab+huawei@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>