Doc: Adding landing page names to navigation variables.

-used by the navigation bar in the HTML output

Change-Id: If4689381fb01c0a6fe8adbd966ead1377942a101
Reviewed-by: Martin Smith <martin.smith@digia.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
Reviewed-by: Venugopal Shivashankar <venugopal.shivashankar@digia.com>