🐛 FIX: linkify link rendering

[chrisjsewell]

Linkify links should always be treated as external links,
and the child text should be rendered (rather than using the href)

[codecov]

Codecov Report

Merging #499 (9541170) into master (574b525) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #499   +/-   ##
=======================================
  Coverage   89.81%   89.82%           
=======================================
  Files          16       16           
  Lines        2072     2074    +2     
=======================================
+ Hits         1861     1863    +2     
  Misses        211      211           

Flag	Coverage Δ
pytests	89.82% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
myst_parser/docutils_renderer.py	91.63% <100.00%> (+0.02%)	⬆️

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