docs: add configuration policy guidance

[ADITYA-CODE-SOURCE]

Fixes #8300.

Why

• We have an established convention around adding sys props / env vars and keeping declarative config aligned, but it is not documented in the knowledge base.
• Contributors need a clear place to look when proposing configuration changes.

What

• add docs/knowledge/configuration-policy.md documenting the configuration policy
• document that new env vars / sys props should be added judiciously
• document that declarative config should remain a strict superset of env/sysprop configuration
• add the new guidance to docs/knowledge/README.md so it is discoverable from the knowledge index

Testing

• ./gradlew --no-daemon --max-workers=1 -Dorg.gradle.jvmargs="-Xmx512m -XX:MaxMetaspaceSize=192m" spotlessCheck

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.00%. Comparing base (2f1d950) to head (b2b0f95).
⚠️ Report is 13 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #8429      +/-   ##
============================================
+ Coverage     90.96%   91.00%   +0.04%     
- Complexity     7809     7817       +8     
============================================
  Files           892      893       +1     
  Lines         23702    23721      +19     
  Branches       2361     2364       +3     
============================================
+ Hits          21561    21588      +27     
+ Misses         1420     1410      -10     
- Partials        721      723       +2     

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[ADITYA-CODE-SOURCE]

@psx95 @jack-berg Updated — moved guidance into api-design.md, addressed all wording nits.
Rebased onto latest main and re-ran spotlessCheck.

[zeitlinger]

I think by default - new config options should be declarative config only

[ADITYA-CODE-SOURCE]

@jack-berg @zeitlinger Updated the configuration section to follow the paragraph-style guidance, removed the PR boilerplate, incorporated declarative config as the default priority interface for new non-programmatic options, and re-ran spotlessCheck.

[ADITYA-CODE-SOURCE]

@psx95 Added a short note recommending an issue or discussion post when proposing new env var / system property additions. I also re-ran spotlessCheck.

[jack-berg]

Thanks @ADITYA-CODE-SOURCE

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