Add clear link back to getodk.org

As we think about reworking the main site, we want to have a clear link back to the getodk.org from the docs. Perhaps we use the same header? Or add links to the existing footer?

Start by locating the shared header and footer templates in the Sphinx documentation, then review how navigation links are currently presented. Choose a clear link placement back to getodk.org; done means the link is visible and works from the relevant documentation pages.

Written by the indexing model from the issue text.