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

Documentation/vm/index.rst

@@ -31,6 +31,7 @@ descriptions of data structures and algorithms.
    active_mm
    balance
    cleancache
+   free_page_reporting
    frontswap
    highmem
    hmm