Rework CONTRIBUTING.md, extracting bits to new /docs/knowledge/* for …

[jack-berg]

…improved human and agent experience

Motivated by work from @trask and friends on introducing https://github.com/open-telemetry/opentelemetry-java-instrumentation/tree/main/.github/agents/knowledge, and interesting tools to automatically apply conventions to individual modules.

Also, we recently introduced a simple AGENTS.md in #8174. On that PR, I commented that we're long over due for a modernization of CONTRIBUTING.md.

This PR lays the foundation:

• Rescope CONTRIBUTING.md to cover project process and scope.
• Extract the bits related to coding patterns, build conventions, testing guidance, and API stability to new docs/knowledge
• Structure docs/knowlege to be consumable by humans and machines:
  • For humans, these documents use plain language and examples rather than rigid rules. Where
    something is a strong convention, it's described as such. Where something is a matter of
    judgment, it's described that way too.
  • For machines. be sensitive to context window, and structure the documents so machines can load only files relevant to the relevant scope. docs/knowledge/README.md acts as an index to individual documents, with instructions agents can use to determine when to load.
• Update AGENTS.md to follow instructions from docs/knowledge/README.md
• For the most part, just restructure content rather than adding new (planning on opening followup PRs). However, I did do some lightweight housekeeping to remove content that was plain wrong.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.26%. Comparing base (f669d31) to head (8b1bde4).

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #8303   +/-   ##
=========================================
  Coverage     90.26%   90.26%           
  Complexity     7688     7688           
=========================================
  Files           850      850           
  Lines         23198    23198           
  Branches       2354     2354           
=========================================
  Hits          20940    20940           
  Misses         1531     1531           
  Partials        727      727           

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