docs: move nommu-mmap.txt to admin-guide and rename to ReST

The nommu-mmap.txt file provides description of user visible
behaviuour. So, move it to the admin-guide.

As it is already at the ReST, also rename it.

Suggested-by: Mike Rapoport <[e-mail]>
Suggested-by: Jonathan Corbet <[e-mail]>
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>
Link: https://lore.kernel.org/r/3a63d1[e-mail]
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/admin-guide/mm/index.rst

···
    idle_page_tracking
    ksm
    memory-hotplug
+   nommu-map
    numa_memory_policy
    numaperf
    pagemap

Documentation/admin-guide/sysctl/vm.rst

···
 
 The default value is 1.
 
-See Documentation/nommu-mmap.txt for more information.
+See Documentation/admin-guide/mm/nommu-mmap.rst for more information.
 
 
 numa_zonelist_order

Documentation/gpu/drm-mm.rst

···
 a pointer on drm_gem_cma_get_unmapped_area().
 
 More detailed information about get_unmapped_area can be found in
-Documentation/nommu-mmap.txt
+Documentation/admin-guide/mm/nommu-mmap.rst
 
 Memory Coherency
 ----------------

init/Kconfig

···
 	  userspace.  Since that isn't generally a problem on no-MMU systems,
 	  it is normally safe to say Y here.
 
-	  See Documentation/nommu-mmap.txt for more information.
+	  See Documentation/mm/nommu-mmap.rst for more information.
 
 config SYSTEM_DATA_VERIFICATION
 	def_bool n

mm/Kconfig

···
 	  This option specifies the initial value of this option.  The default
 	  of 1 says that all excess pages should be trimmed.
 
-	  See Documentation/nommu-mmap.txt for more information.
+	  See Documentation/mm/nommu-mmap.rst for more information.
 
 config TRANSPARENT_HUGEPAGE
 	bool "Transparent Hugepage Support"

mm/nommu.c

···
  *  Replacement code for mm functions to support CPU's that don't
  *  have any form of memory management unit (thus no virtual memory).
  *
- *  See Documentation/nommu-mmap.txt
+ *  See Documentation/mm/nommu-mmap.rst
  *
  *  Copyright (c) 2004-2008 David Howells <[e-mail]>
  *  Copyright (c) 2000-2003 David McCullough <[e-mail]>