Use trunk instead of just for WebGL example

[campbellcole]

Currently, the WebGL example is compiled using the justfile. This works, but for a beginner it may be less reproducible due to how involved the justfile configuration is.

Without any changes to the code or configuration, trunk can be used to enable hot reloading and automatic serving of the built files:

cd examples/webgl
cargo install trunk
trunk serve --open

trunk can also be used to build the example without serving: trunk build [--release]

The only thing that needs changing are a few lines in the documentation, and perhaps abridging the justfile