Indicate external links with an external link icon

[stevepiercy]

Consider adding an indicator for external links with an external link icon.

Current

Proposed

Implementation

Stolen from PyData Sphinx Theme.

.reference.external::after {
  margin-left: 0.3em;
  display: inline-block;
  content: var(--pst-icon-external-link);
  white-space: nowrap;
  font: var(--fa-font-solid);
  font-size: .75em;
}