docs: Added icon on external links

[applejag]

I was browsing the docs and got confused by some links sending me to wikipedia thought "hmm I wish the docs had that icon thing to show which links are external links". A quick google showed that vitepress has this built in.

Before:

After:

---

It would be much nicer to have something like the favicon to the page next to the link, you know like this:

(https://css-tricks.com/favicons-next-to-external-links/)

but I couldn't find a quick solution for that with vitepress online and didn't have the energy to dig much further so I just settled for the built in feature.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.92%. Comparing base (3ad3902) to head (831508f).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #184   +/-   ##
=======================================
  Coverage   95.92%   95.92%           
=======================================
  Files          49       49           
  Lines        3144     3144           
=======================================
  Hits         3016     3016           
  Misses        101      101           
  Partials       27       27           

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[jmattheis]

👍, the builtin feature is good enough.