test: check stability block position in API markdown

[Renegade334]

The doc generator will only add a stability marker to the index if the blockquote element appears close to the relevant heading. There is an ordering convention (<heading> [<yaml>] <stability>) which is not currently enforced.

Adding a test for this identifies one element which is currently non-compliant, buf.slice() in buffer.md, which does not appear as deprecated in the index due to the malpositioned deprecation block.

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 90.20%. Comparing base (641653b) to head (91a8078). :warning: Report is 1139 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #58590      +/-   ##
==========================================
- Coverage   90.21%   90.20%   -0.01%     
==========================================
  Files         635      635              
  Lines      187494   187495       +1     
  Branches    36838    36818      -20     
==========================================
- Hits       169144   169129      -15     
+ Misses      11145    11117      -28     
- Partials     7205     7249      +44     

see 49 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/70187/