fix(docs): add build path to nodePaths for colocated hooks

[benvp]

Since the introduction of colocated hooks in v1.8 the example for a custom build.js file requires the build path to be present as well. If it is not present, the build of the assets will fail with a resolve error Could not resolve "phoenix-colocated/<app>"

The default :esbuild config already includes the build path by using Mix.project.build_path()

[SteffenDE]

I believe for that to work, you'd also need to adjust the watcher to explicitly pass env: %{"MIX_ENV" => to_string(Mix.env())} and also pass MIX_ENV in the configured aliases.

Otherwise, unless you manually set MIX_ENV, it won't be set in the build script.