Allow authors to add extra links to the top of pages

[choldgraf]

Many themes allow users to configure some extra external links that are in the documentation. For example, the pydata sphinx theme adds "social links" to the top right:

In Jupyter Book you can add external links to your _toc.yml file, but this feels like a different use-case from the one I describe above. I think we could use the top-bar for something like this, but the question is whether it's worth supporting.

Do we want to support people adding:

• extra, arbitrary links to the top-bar (perhaps with a "link icon" dropdown)
• special outside links like "Twitter" or "GitHub" with their icons

There's a prototype for links here: executablebooks/sphinx-book-theme#97

demo: https://297-244496643-gh.circle-artifacts.com/0/html/index.html

would love to know what folks think