infra: configure CODEOWNERS and branch protection

[marc0olo]

Context

Set up CODEOWNERS so PRs automatically request review from the right team members, and enable branch protection on main.

Steps

1. Create .github/CODEOWNERS mapping doc sections to reviewers:
   • docs/guides/chain-fusion/ → chain fusion team
   • docs/languages/motoko/ → Motoko team
   • docs/languages/rust/ → Rust CDK team
   • General catch-all → docs team
2. Enable branch protection on main:
   • Require PR review before merging
   • Require CI checks to pass (once infra: set up CI workflows #116 is done)
   • Prevent force pushes to main

Depends on

• CI workflows (infra: set up CI workflows #116) — branch protection should require CI to pass