Add cppreference link to ignore list

[urfeex]

The workflow gets a 403 on this

---

Note

Low Risk
Low risk: CI-only change that just expands the lychee --exclude list to avoid a known 403, with no impact on runtime code or artifacts.

Overview
Updates the Check Links GitHub Actions workflow to exclude a specific cppreference.com URL from lychee link checking, preventing CI failures caused by 403 responses.

^{Reviewed by Cursor Bugbot for commit 849aa54. Bugbot is set up for automated code reviews on this repo. Configure here.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.56%. Comparing base (8f35234) to head (849aa54).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #506      +/-   ##
==========================================
- Coverage   76.96%   76.56%   -0.41%     
==========================================
  Files         116      116              
  Lines        6342     6340       -2     
  Branches     2761     2761              
==========================================
- Hits         4881     4854      -27     
- Misses       1112     1135      +23     
- Partials      349      351       +2     

Flag	Coverage Δ
python_scripts	75.90% <ø> (ø)
start_ursim	82.72% <ø> (-2.32%)	⬇️
ur20-latest	72.93% <ø> (-0.16%)	⬇️
ur5e-10.11.0	66.76% <ø> (-0.44%)	⬇️
ur5e-10.12.0	68.23% <ø> (-0.11%)	⬇️
ur5e-10.7.0	66.33% <ø> (-0.14%)	⬇️
ur5e-5.9.4	73.51% <ø> (-0.22%)	⬇️

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