Restore functionality for external link indicators

[agriyakhetarpal]

#162 and #163 had previously added an external link indicator icon to the theme, which was later reverted in #269 due to discussion in ##233 (comment), where it was pointed out that it was broken for dark mode and there were some questions around how to style SVGs. Also, it was mentioned that this functionality was not being used in the websites that use the theme. However, we had enabled it downstream and it caught my eye on a routine visit recently. I've put forward in this PR a more correct implementation for external link indicators, drawing from how the PyData Sphinx Theme does it.

This also works with light and dark mode, the rationale being that the ::after pseudo-element doesn't explicitly set a color property and inherits it from the parent – wherein, in this case, we have --pst-color-text-base and --pst-color-link-hover that are defined with different values for light and dark mode already.

cc: @stefanv

Here are some screenshots for your reference:

Location	Colour mode	Screenshot
Navbar	Light
Navbar	Dark
Footer	N/A

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