Remove broken link in README for mirador-start

[marlo-longley]

Closes #4224

This seems like it would have been cool for users. I never saw it in action.

Yes this is my first vibecode, check it out! https://deploy-preview-4250--mirador-dev.netlify.app/playground
If this feature is of interest I can make it nicer. Just an idea.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.5%. Comparing base (906f5eb) to head (ffca551).

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #4250     +/-   ##
=======================================
+ Coverage   95.5%   95.5%   +0.1%     
=======================================
  Files        328     328             
  Lines      16370   16370             
  Branches    2590    2595      +5     
=======================================
+ Hits       15617   15622      +5     
+ Misses       748     744      -4     
+ Partials       5       4      -1     

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