Clarify how to contribute, on the website itself

[mortenfyhn]

Hi, I just opened #82 but it took me a moment to figure out how to contribute to the docs / where the repo was.

How about some kind of visible link to this repo somewhere on the site?

I have a very simple experiment here: https://github.com/mortenfyhn/developer.rebble.io/tree/github-link. It adds a GitHub link to the sidebar:

and

The wide sidebar variant makes the sidebar slightly wider than before, which is not ideal. In any case, I'm happy to hack some variant of this and open a PR.