Demo: Add link to demo source code

### Problem

The Granite Demo is a massive help for me in learning both Vala, doing UI with Vala, and discovering what Granite can do.

However valadoc is very... theoretical, and it happens often for me to wonder how something is used concretely
or see something in the demo, and think "oh! I like this! I want to use it"

### Proposal

Add in the granite-demo somewhere accessible a link to the source code on github
it could be that each view has a link sending directly to the corresponding file in github
OR INSTEAD a popover that displays the code for that page, or switches the page view to its code

So anytime i see cool stuff i can immediately check how is it implemented, instead of firing up firefox and having to navigate here.

### Prior Art (Optional)

_No response_