Docs: retry ui-bundle download

[sdarwin]

Sometimes the superproject build fails because of a remote download. This pull request adds retries to the antora ui-bundle download.
On a related topic, very recently Joaquin updated boostorg/unordered, removing duplicate copies of the documentation, and he has another plan to remove the node_modules folder after docs builds.
Keep an eye on boostorg/unordered, copy these modifications over.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.02%. Comparing base (e0ead3b) to head (8f003b5).
⚠️ Report is 9 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #60      +/-   ##
===========================================
- Coverage    92.03%   92.02%   -0.01%     
===========================================
  Files           40       40              
  Lines         2838     2835       -3     
  Branches      1354     1354              
===========================================
- Hits          2612     2609       -3     
  Misses         187      187              
  Partials        39       39              

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[cppalliance-bot]

An automated preview of the documentation is available at https://60.openmethod.prtest3.cppalliance.org/libs/openmethod/doc/html/index.html

If more commits are pushed to the pull request, the docs will rebuild at the same URL.

2026-01-06 15:06:33 UTC

[sdarwin]

Update: the newest doc/Jamfile from unordered succeeds in removing node_modules.