Fix stale api-stability.md reference in knowledge README

[thswlsqls]

Description

• docs/knowledge/README.md line 26 cites a deleted file api-stability.md as the filename-pattern example.
• PR Add guidance for null checking, promote ApiUsageLogger to opentelemetry-common public API #8318 (commit 3f3780c) merged api-stability.md into api-design.md and updated the topics table link (line 18) but missed this prose example in the same file — an incomplete fix.
• Replaces the single stale token api-stability.md with api-design.md; testing-patterns.md still exists and is left unchanged.
• Restores consistency within the file: the table and the prose example now both point to api-design.md.
• Related: Add guidance for null checking, promote ApiUsageLogger to opentelemetry-common public API #8318

Testing done

• Docs-only change; no behavior change, so build/test verification is exempt (Gradle skipped per docs-only policy).
• Verified zero residual references: grep -rn "api-stability" docs/ returns no matches.
• Confirmed docs/knowledge/api-design.md exists.
• No public API change (no apidiff) and not user-facing (contributor docs), so no CHANGELOG entry.

[linux-foundation-easycla]

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• ✅ login: thswlsqls / name: Eunbin Son (4620cd9)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.77%. Comparing base (824334c) to head (4620cd9).

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #8491   +/-   ##
=========================================
  Coverage     78.77%   78.77%           
  Complexity     8579     8579           
=========================================
  Files          1009     1009           
  Lines         28993    28993           
  Branches       3599     3599           
=========================================
  Hits          22839    22839           
  Misses         5311     5311           
  Partials        843      843           

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[thswlsqls]

/easycla

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