Write dev/contributor docs

[seanmonstar]

• [x] Update CONTRIBUTING.md
  • [x] Move most content into dev folder documents
• [ ] Make docs folder with developer docs
  • [ ] README as an index file
  • [x] Charter and Tenets
  • [x] Conduct
  • [x] Roadmap
  • [x] Issues, labels
  • [ ] Pull requests
  • [x] Committing
  • [ ] Testing
  • [x] MSRV (Minimum Supported Rust Version)
  • [ ] Release Process
  • [ ] Design
    • Feature flags
    • private-in-public types as impl Trait
    • Poll functions vs async
  • [x] Governance
• [x] Render dev guide on https://hyper.rs/contrib