docs(readme): use standard relative paths for license, code of conduct, and contributing guidelines

[loulanyue]

Summary

• Replaces hardcoded branch URLs (https://github.com/vercel/next.js/blob/canary/...) in README.md with standard relative Markdown links for license.md and CODE_OF_CONDUCT.md.
• Changes absolute leading path /contributing.md to relative contributing.md.

[vercel]

Suggested change

	Do note that our [Code of Conduct](CODE_OF_CONDUCT.md) applies to all Next.js community channels. Users are **highly encouraged** to read and adhere to it to avoid repercussions.
	## Contributing
	Contributions to Next.js are welcome and highly appreciated. However, before you jump right into it, we would like you to review our [Contribution Guidelines](/contributing.md) to make sure you have a smooth experience contributing to Next.js.
	Contributions to Next.js are welcome and highly appreciated. However, before you jump right into it, we would like you to review our [Contribution Guidelines](contributing.md) to make sure you have a smooth experience contributing to Next.js.
	Do note that our [Code of Conduct](../../CODE_OF_CONDUCT.md) applies to all Next.js community channels. Users are **highly encouraged** to read and adhere to it to avoid repercussions.
	## Contributing
	Contributions to Next.js are welcome and highly appreciated. However, before you jump right into it, we would like you to review our [Contribution Guidelines](../../contributing.md) to make sure you have a smooth experience contributing to Next.js.

Relative Markdown links to CODE_OF_CONDUCT.md and contributing.md in packages/next/README.md are broken because those files only exist at repo root, not in packages/next/