Add a link from each extensions documentation page to its GitHub repo

[duncandewhurst]

I use the extension explorer when I want to check something in an extension's schema or documentation.

Sometimes I also want to check back to the actual Github repository for the extension, for example to look at the commit history or to get the url to check the repository out for editing.

I've just discovered I can get there via the 'Maintained by...' link, but it would be easier if there was a direct link.

[jpmckinney]

This can be done – the logic to determine repository URLs from 'Base URL' is implemented in extension_registry.py.