docs: align README/Antora examples

[alandefreitas]

The transition from quickbook to antora created two sources of truth for testing code snippets. We fix this and include missing snippets in the tests.

fix #967

[cppalliance-bot]

An automated preview of the documentation is available at https://968.url.prtest2.cppalliance.org/index.html

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

2026-01-27 05:29:50 UTC

[cppalliance-bot]

GCOVR code coverage report https://968.url.prtest2.cppalliance.org/gcovr/index.html
LCOV code coverage report https://968.url.prtest2.cppalliance.org/genhtml/index.html
Coverage Diff Report https://968.url.prtest2.cppalliance.org/diff-report/index.html

Build time: 2026-01-27 15:47:57 UTC

[codecov]

Codecov Report

❌ Patch coverage is 98.42105% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.14%. Comparing base (14fb347) to head (25bd3ce).
⚠️ Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
doc/modules/ROOT/examples/unit/snippets.cpp	98.42%	6 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #968      +/-   ##
===========================================
- Coverage    99.35%   99.14%   -0.21%     
===========================================
  Files          161      162       +1     
  Lines         8771     9937    +1166     
===========================================
+ Hits          8714     9852    +1138     
- Misses          57       85      +28     

Files with missing lines	Coverage Δ
include/boost/url/format.hpp	100.00% <ø> (ø)
include/boost/url/url_view_base.hpp	100.00% <ø> (ø)
doc/modules/ROOT/examples/unit/snippets.cpp	97.25% <98.42%> (ø)

... and 1 file with indirect coverage changes

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