Improve appearance of external links in built-in docs

[Mickeon]

Closes https://github.com/godotengine/godot-proposals/issues/9369

This PR makes the external links in the built-in class reference:

• Display with an "external link" icon
• Coloured differently than surrounding text
• Without outline unless hovered on
• With a tooltip containing the link

I'm unconvinced on the actual color, though. See also a RocketChat thread. The gist of it is that, I feel like the excessive accent colour brings too much attention to external links, and the icon exists too. Links to the built-in class reference are more important in the editor and should be more notable, as well.

Normal Link Color	Saturated Link Color

It's also currently not possible to hide the hint tag's underline individually. Hence the dotted underline on these links.

[tetrapod00]

I think the bikeshed should be normal link colored.

[Mickeon]

Updated the PR to also reduce code duplication.