docs/core-api: split memory management API to a separate file

This is basically copy-paste of the memory management section from
kernel-api.rst with some minor adjustments:

* The "User Space Memory Access" is moved to the beginning
* The get_user_pages_fast reference is now a part of "User Space Memory
  Access"
* And, of course, headings are adjusted with section being promoted to
  chapters

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Mike Rapoport <[email protected]>
Reviewed-by: Andrew Morton <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Michal Hocko <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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