validate documentation in #[zenoh_macros::unstable] to avoid broken text

[milyin]

The macro #[zenoh_macros::unstable] adds notification that the API is unstable after first line of the documentation.
This causes 2 problems:

• if first line is not logically separate from other text, the text is broken with this "unstable notice" panel
• the rustdoc don't parse documentation links which immediately follows the html-formatted panel

Both problems can be fixed by adding empty line after fist line of documentation. This logically separates first statement, then the "unstable notice" panel is inserted and other documentation with correctly parsed links follows.

This fix makes #[zenoh_macros::unstable] display error if this rule is not followed

---

🏷️ Label-Based Checklist

Based on the labels applied to this PR, please complete these additional requirements:

Labels: enhancement

✨ Enhancement Requirements

Since this PR enhances existing functionality:

• Enhancement scope documented - Clear description of what is being improved
• Minimum necessary code - Implementation is as simple as possible, doesn't overcomplicate the system
• Backwards compatible - Existing code/APIs still work unchanged
• No new APIs added - Only improving existing functionality
• Tests updated - Existing tests pass, new test cases added if needed
• Performance improvement measured - If applicable, before/after metrics provided
• Documentation updated - Existing docs updated to reflect improvements
• User impact documented - How users benefit from this enhancement

Remember: Enhancements should not introduce new APIs or breaking changes.

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[github-actions]

PR missing one of the required labels: {'ci', 'bug', 'dependencies', 'enhancement', 'internal', 'new feature', 'api-sync', 'breaking-change', 'documentation'}

[codecov]

Codecov Report

❌ Patch coverage is 77.55102% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.93%. Comparing base (f9d6b39) to head (8e55cab).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
commons/zenoh-macros/src/lib.rs	76.08%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2348      +/-   ##
==========================================
+ Coverage   71.88%   71.93%   +0.05%     
==========================================
  Files         387      387              
  Lines       61972    61986      +14     
==========================================
+ Hits        44546    44590      +44     
+ Misses      17426    17396      -30     

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[github-actions]

PR missing one of the required labels: {'new feature', 'api-sync', 'ci', 'internal', 'enhancement', 'breaking-change', 'dependencies', 'documentation', 'bug'}

[github-actions]

PR missing one of the required labels: {'breaking-change', 'new feature', 'internal', 'documentation', 'enhancement', 'bug', 'api-sync', 'dependencies', 'ci'}