docs: fix broken CODE_OF_CONDUCT and developer setup links in README

[ethanlin01x]

Closes #837
Fix two 404 links (Code of Conduct, developer setup guide) in the README "Contributing" section.

Changes

• Point the developer setup guide link to docs/contributing/setup.rst.
• Point the Code of Conduct link to the ASF Code of Conduct policy page (https://www.apache.org/foundation/policies/conduct.html), consistent with docs/asf/index.rst.

How I tested this

• Verified both old links return HTTP 404 on current main.
• Verified both new links return HTTP 200.
• Docs-only change — no code or functionality affected.

Notes

Checklist

• PR has an informative and human-readable title (this will be pulled into the release notes)
• Changes are limited to a single goal (no scope creep)
• Code passed the pre-commit check & code is left cleaner/nicer than when first encountered.
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future TODOs are captured in comments
• Project documentation has been updated if adding/changing functionality.

[elijahbenizzy]

thanks!