Docs/mm/index: move allocation profiling document to unsorted documents chapter

The memory allocation profiling document was added to the bottom of the
new outline.  Apparently it was not decided by well-defined guidelines
or a thorough discussions.  Rather than that, it was added there just
because there was no place for such unsorted documents.  Now there is
the chapter.  Move the document to the new place.

Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240701190512.49379-5-sj@kernel.org