Update the docs on the third-party-dependencies section to describe dynamic import method

The [docs for third party dependencies](https://ember-fastboot.com/docs/addon-author-guide#third-party-dependencies) describes a rather old technique that involves automatically wrapping your vendor libs in a "fastboot guard". This method a) won't work for anything that is using ember-auto-import and b) won't work in a broccoli-free embroider build

We should update this section to describe the fact that you can use dynamic imports inline with your code that has a fastboot guard on it. This is idiomatic in terms of the wider JS community i.e. don't import things that you know won't work in your environment