Document how to minimize JS when using `aio app:deploy`

Forking @moritzraho 's answer here to be documented --

Developer can replace our build script with a build-static hook in the package.json scripts field. https://www.adobe.io/apis/experienceplatform/project-firefly/docs.html#!AdobeDocs/project-firefly/master/guides/app-hooks.md

We did not minify by default because this was introducing bugs in some cases. However, we do tree shaking (remove uneeded code in deps) and this has shown to reduce the size already enough in most cases.

Read the existing app-hooks.md guidance and inspect the package.json scripts field in the context of aio app:deploy. Document how a developer can use a build-static hook to minimize JavaScript, including the existing tree-shaking behavior and the reason minification is not enabled by default; done when the guidance is clear and linked from the relevant documentation.

Written by the indexing model from the issue text.