Add links to github to every page of docs

Is your feature request related to a problem? Please describe.

My company is using alembic and it's great! I occasionally have to look at the docs, and then want to jump to GitHub to either look for issues or check the source code. Currently I either have to manually enter the URL and hope my browser can autocomplete, or do a web search, or go to the "Front Matter" page (which requires going to the top level from a nested page).

Describe the solution you'd like
A lot of projects have important links in the side bar, e.g. via https://ghbtns.com or https://github.blog/2008-12-19-github-ribbons/ . In a project I worked on, we added them to the layout template: https://github.com/stellargraph/stellargraph/blob/5ca1e59e91cb6ac470bf19ff3da39b3a1a68650e/docs/_templates/layout.html (visible at https://stellargraph.readthedocs.io/en/stable/)

These links would appear on all pages, and thus be make it very easy for people to jump here!

Describe alternatives you've considered

Any other form of links that automatically apply on each page would be fine.

Additional context

Here's what it looks like at the links above (at the time of writing):

Start by reviewing the documentation layout and the referenced docs/_templates/layout.html example to identify where a site-wide link can be added. Check the generated documentation pages and confirm that a GitHub link appears from both top-level and nested pages.