Support for icon links for social, github, external links, etc

[choldgraf]

It would be useful if MyST sites could link out to other websites via the theme's user interface. This would allow communities to define other external links that are relevant across the entire site.

It is common for links like this to be represented with small, standardized icons, and to provide users the ability to both choose the icon and the external link.

This often goes in either the navigation bar at the top, or the primary sidebar.

Examples in the wild

The mystmd.org website implements its own custom icon links widget in the top-right (these are hidden on mobile)

The Sphinx Book Theme implements icon links in the left sidebar (these are visible on mobile in the left drawer)

The PyData Sphinx Theme implements icon links in the navigation bar (these are visible on mobile in the left drawer)