Add a focused contributor guide

[SihanTeng]

Summary

• Add the verified build, test, browser, and pull-request guidance from Use path-component semantics for crawl controls #76 as a standalone contribution.
• Remove the undefined reference to review notes.
• Remove the PR feat: send cookies with every request during a clone #58-specific authentication diagnosis from the guide; that feedback now lives on feat: send cookies with every request during a clone #58 itself.

Why

The contributor guide is useful, but it is independent of crawl-control behavior and was requested as its own PR.

Test plan

• Documentation-only change; commands and environment variables were verified during the Use path-component semantics for crawl controls #76 review.

[tamnd]

I went through this claim by claim against the tree and everything holds, including the Windows CI suggestion which is still live since our matrix is only ubuntu and macos. The one concern per PR line landed well given you then split #74 and #76 into seven focused PRs, so thanks for that. Merging.