Adds help menu entry for launching documentation resources.

* Body of new menu is defined in startup/gui as this content appears specific to main app
* Declares a placeholder for example-script launching, ready for future implementation.
* uses webbrowser module to perform launching as my research suggests that is the most platform independent approach.
** for best results users would need to set $BROWSER env var, otherwise it's a bit of a gamble what app launches the files
** also, the pdf files need to be placed in GAFFER_ROOT/doc as part of the install process.