Documentation: igb: Prepare documentation for RST conversion

Before making the conversion to the RST (reStructured Text) format, there
are changes needed to the documentation so that there are no build errors.

Also fixed old/broken URLs to the correct or updated URL.
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>