Add CONTRIBUTING.md for each repo #2

Description

The org profile README (profile/README.md) tells contributors to "see each repository's CONTRIBUTING.md for specifics", but none of the repos (docsgraph, docsgraph-server, docsgraph-eval) have one yet.

Proposed Solution

Add a CONTRIBUTING.md to each repo covering local setup, how to run tests, branch/PR conventions, and the effort/label conventions already in use (see ISSUE_TEMPLATE and PULL_REQUEST_TEMPLATE.md in this repo). Optionally add a shared base CONTRIBUTING.md here in .github that repo-specific ones can link back to.

Acceptance Criteria

• docsgraph, docsgraph-server, and docsgraph-eval each have a CONTRIBUTING.md
• Each covers local setup and how to run tests
• Each documents the effort-label and PR checklist conventions
• Links from profile/README.md resolve to real content