[docs]: Add GitHub link on homepage

# Documentation request

Is it really that hard to put a link to this repo somewhere sensible?

Search cloud init and find the docs, https://cloudinit.readthedocs.io/en/latest/index.html.
"Project and community" > "Contribute on GitHub"
Oh it links to another page.
> Cloud-init is hosted and managed on [GitHub](https://github.com/). If you’re not familiar with how GitHub works, their [quickstart documentation](https://docs.github.com/en/get-started/quickstart) provides an excellent introduction to all the tools and processes you’ll need to know.

Oh, the GitHub link does not take me to the repo.
Spend 10 minutes clicking through links on the page and looking for a link to the repo.
Give up.
Click "Give Feedback".
Finally, even if not to repo homepage.

---
*Reported from: https://cloudinit.readthedocs.io/en/latest/search.html?q=github.com