doc: auto-generate date in guides

doc: auto-generate date in guides

The index.html file for each of the "guide" docs had a hard-coded
date value in them of June 2014. Rather than update each of these
for each revision, just use the |today| directive to insert the date
at which the document was generated.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Bernard Iremonger <bernard.iremonger@intel.com>