[DOC] update ABI policy docs to clarify breaking changes

[dbarker]

Fixes # (issue)

Updates the ABI policy documents to expand on allowed changes and to cover API header implementation changes that impact observable behavior (clarification based on #4174).

Changes

• Update the policy to highlight the case where an application links to multiple instrumented libraries (where each may include API headers from different otel-cpp versions).
• Add note and link to the stdlib doc for when building with c++ standard types.
• Provide examples of API header changes that may be allowed in a stable version and what must target an experimental version
• Provide example and guidance on observable behavior changes (changes to method implementations) and requires these to be targeted to the experimental ABI version due to potential ODR violations.
• Updates the ABI version policy to point to CMake and Bazel files as the source of truth for the current stable and experimental ABI version

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.98%. Comparing base (82a2ada) to head (a9181ba).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4180   +/-   ##
=======================================
  Coverage   82.98%   82.98%           
=======================================
  Files         406      406           
  Lines       17264    17264           
=======================================
  Hits        14325    14325           
  Misses       2939     2939           

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[lalitb]

LGTM overall. Thanks for tightening the ABI guidance.