Add a link to the extension shortcuts page

[Simounet]

@Simounet: If you want to implement a link, there should be something like openShortcutSettings() based on this fix in Firefox 137:
https://hg-edge.mozilla.org/integration/autoland/rev/f197845785d6

Originally posted by @HolgerAusB in #499

Here is the API doc only implemented in Firefox at the moment.