Movement of groovy website

[Paul King <pa...@asert.com.au>]

The source code for the groovy website has moved to:

https://github.com/apache/groovy-website/

Currently the generated content is still hosted off-site but that will move
over the next few weeks too. I'll wait until the move to gitbox has been
completed since we need to update the release process in parallel.

You shouldn't notice any changes to the website itself from either change.
Your PR requests for changes just need to occur on the new repo site.

Cheers, Paul.