doc: add permalinks to document headings

Permalinks make it much easier to create html links to specific sections
in a document (you over over the section heading and an icon appears that
you can click on to get a reference link directly to that section).

Change-Id: I905449158f834783b9df6c04bf5fa3e5f245b3fd
Signed-off-by: David B. Kinder <david.b.kinder@intel.com>