adds 'This site is powered by Netlify.' (#797)

This PR adds a small link back to Netlify in the footer (specifically, in the user-editable config); this is a necessary prerequisite for the [Netlify OSS Plan](https://www.netlify.com/legal/open-source-policy). 

See: https://github.com/orgs/just-the-docs/teams/maintainers/discussions/1/comments/18.