docs: fix snippet-preamble notes and io_uring platform detail

[sgerbino]

• Wrap the "Code snippets assume" preamble in the hash-server and reconnect tutorials in a [NOTE] block so the snippet renders inside the admonition, matching the other pages
• io_context platform details: io_uring is an implemented Linux backend, not planned

[cppalliance-bot]

An automated preview of the documentation is available at https://287.corosio.prtest3.cppalliance.org/index.html

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

2026-06-21 16:18:58 UTC

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.90%. Comparing base (c05c43b) to head (ce5be06).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #287   +/-   ##
========================================
  Coverage    77.90%   77.90%           
========================================
  Files           96       96           
  Lines         7160     7160           
  Branches      1751     1751           
========================================
  Hits          5578     5578           
  Misses        1078     1078           
  Partials       504      504           

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[cppalliance-bot]

GCOVR code coverage report https://287.corosio.prtest3.cppalliance.org/gcovr/index.html
LCOV code coverage report https://287.corosio.prtest3.cppalliance.org/genhtml/index.html
Coverage Diff Report https://287.corosio.prtest3.cppalliance.org/diff-report/index.html

Build time: 2026-06-21 16:28:18 UTC