👌 Add external link icon to external links in sidebar

[jorisvandenbossche]

This adds the "external link" icon to external links in the sidebar. We already have this for the navbar (actually using a specific configuration option to add those, while we could probably also do that through sphinx' toctree, but that's for another PR). But this adds it for the sidebar as well (eg like sphinx-book-theme and furo do as well).

In sphinx-book-theme we explicitly add the icon to the html, like we do for the navbar here as well (using beautifulsoup, see https://github.com/executablebooks/sphinx-book-theme/blob/39aaaa3e4d908fe7be8b7378ad474d8c6b86aa16/sphinx_book_theme/__init__.py#L126-L130). But in this PR I took the approach of Furo to add ::after the link with CSS, but using the same icon/styling we already use in this theme / sphinx-book-theme.

[jorisvandenbossche]

Result:

[choldgraf]

This sounds like a good idea to use the css approach! Agreed we should do the same for the navbar and remove the custom beautiful soup bits as much as possible