Link to the example YAML file in `docs/contributing_libraries.md` is 404

Current:

<a href="src/content/libraries/en/p5.warp.yaml">copying an existing library</a>

This resolves relative to the current document and points to:

https://github.com/processing/p5.js-website/blob/main/docs/src/content/libraries/en/p5.warp.yaml

which does not exist, causing a 404 error.

It should be:

<a href="/src/content/libraries/en/p5.warp.yaml">copying an existing library</a>

which correctly resolves to:

https://github.com/processing/p5.js-website/blob/main/src/content/libraries/en/p5.warp.yaml