docs: move memory hotplug description into admin-guide/mm

The memory hotplug description in Documentation/memory-hotplug.txt is
already formatted as ReST and can be easily added to admin-guide/mm
section.

While on it, slightly update formatting to make it consistent with the
doc-guide.

Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>