Add Dependabot configuration for automated dependency updates

[aram356]

Summary

Configure GitHub Dependabot to automatically monitor and create PRs for dependency updates across the project's package ecosystems.

Details

The project currently has no automated dependency update mechanism. We should add a .github/dependabot.yml configuration to enable weekly version update checks for:

• Cargo (Rust dependencies) — root / directory
• npm (JS dependencies) — crates/js/lib/ directory
• npm (docs dependencies) — docs/ directory

Acceptance Criteria

• Dependabot configuration file added at .github/dependabot.yml
• Weekly update checks configured for Cargo ecosystem
• Weekly update checks configured for npm ecosystem (crates/js/lib/)
• Weekly update checks configured for npm ecosystem (docs/)