add Dependabot for automated dependency updates

[salmanmkc]

The project doesn't currently have Dependabot configured, so dependency updates (including security patches) require manual tracking. This led me to create PRs such as: #175 and #87, I quite like this project though so decided to investigate more haha.

Adding a .github/dependabot.yml would enable weekly automated PRs for:

• Root npm dependencies (/)
• Docs npm dependencies (/docs)
• GitHub Actions workflow versions

This is a low-effort way to stay on top of security vulnerabilities and keep dependencies current.