Add CONTRIBUTING.md with DCO and PR template

[omghante]

Problem

• Repo only has a short contributing section in README.md, no dedicated guide
• New contributors have to figure out setup, formatting rules, and commit conventions on their own
• No DCO sign-off requirement documented
• No PR template to standardize pull requests

Proposed Changes

• Add CONTRIBUTING.md at repo root covering:
  • Development setup (Java 17, Maven, pre-commit install)
  • Code style rules (tabs, Spotless)
  • Testing requirements
  • DCO sign-off instructions
• Add .github/PULL_REQUEST_TEMPLATE.md with description, related issue, and checklist sections
• Update README contributing section to link to the new CONTRIBUTING.md