Add contributor recognition system

[PAMulligan]

Description

Add an automated contributor recognition system using the All Contributors specification. This publicly recognizes all forms of contribution — code, documentation, design, testing, ideas, and more.

Why

Recognizing contributors encourages continued participation and attracts new contributors. The All Contributors specification is the standard for open source projects and provides a bot for automated updates.

Acceptance Criteria

• Add the `.all-contributorsrc` configuration file
• Add a Contributors section to `README.md` with the all-contributors table
• Set up the @all-contributors bot or GitHub Action
• Configure contribution types relevant to Nerva:
  • `code` — Code contributions
  • `doc` — Documentation
  • `test` — Tests
  • `bug` — Bug reports
  • `ideas` — Ideas and suggestions
  • `review` — Code review
  • `infra` — Infrastructure (CI/CD, deployment)
• Add instructions in `CONTRIBUTING.md` for how contributors are recognized
• CI passes