Docs/mm/index: rename 'Legacy Documentation' to 'Unsorted Documentation'

The intention of 'Legacy Documentation' chapter is to keep the old
documents that not yet sorted into the new outline, and encourage new
documents to be integrated in the new outline from the beginning.

However, the new outline will take some more time to be completed. It
has started about two years ago, and still many parts are not yet
written. Also, there is no clear guidline for placing each document for
all cases, for not only the 'legacy' documents, but also for new
documents. For example, memory allocation profiling document has been
added to the bottom of the new outline. Apparently it was not following
some well-defined guideliens or a result of a discussion.

Furthermore, the title ("legacy") makes people feel the documents on the
chapter might be outdated or not actively maintained.

Rename 'Legacy Documentation' to 'Unsorted Documentation' and remove the
description saying it is for 'older' documents. After this change, new
documents that not clear enough where it should be placed on the new
outline can be added on the chapter while well-defined guidelines or
discussion for the new outline is made.

Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240701190512.49379-4-sj@kernel.org