docs: fix numaperf.rst and add it to the doc tree

Commit 13bac55e ("doc/mm: New documentation for memory performance")
added numaperf.rst, but did not add it to the TOC tree.  There was also an
incorrectly marked literal block leading to this warning sequence:

  numaperf.rst:24: WARNING: Unexpected indentation.
  numaperf.rst:24: WARNING: Inline substitution_reference start-string without end-string.
  numaperf.rst:25: WARNING: Block quote ends without a blank line; unexpected unindent.

Fix the block and add the file to the document tree.

Fixes: 13bac55e ("doc/mm: New documentation for memory performance")
Cc: Keith Busch <keith.busch@intel.com>
Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/mm/index.rst

@@ -31,6 +31,7 @@ the Linux memory management.
    ksm
    memory-hotplug
    numa_memory_policy
+   numaperf
    pagemap
    soft-dirty
    transhuge

Documentation/admin-guide/mm/numaperf.rst

@@ -15,7 +15,7 @@ characteristics.  Some memory may share the same node as a CPU, and others
 are provided as memory only nodes. While memory only nodes do not provide
 CPUs, they may still be local to one or more compute nodes relative to
 other nodes. The following diagram shows one such example of two compute
-nodes with local memory and a memory only node for each of compute node:
+nodes with local memory and a memory only node for each of compute node::
 
  +------------------+     +------------------+
  | Compute Node 0   +-----+ Compute Node 1   |