Reading that Miyazaki book, he talks about basically he writes the treatment, then goes to story boards from there. From what I've heard of Brad Bird, he works similarly, getting into storyboards quickly since animation is inherently visual and we do our editing up front instead of aftwerwards.
Me being the noob that I am, I didn't really know what a treatment is.
Wiki says a treatment's between index cards and an actual script. Basically a quick description of every scene and beat in the story (a beat outline being just a list of beats without descriptions of them), so you have a roadmap of where you need to go when you write the script (or are making the boards).
One way of writing them is to put a big header at the top of each scene, describing the director beats. So it's easy to skim just the headers and get the flow of the story. Here's quick advice on writing one, and here's an article with examples of a prose style treatment vs a header style (skip way to the bottom, the majority of it is satyrical filler about writing treatments)
I also recently came across the idea of director beats. The idea that the director has an overall meaning for each scene. (IE. This scene is about them having to work together despite their animosity, this scene is about the audience getting tense because they see the bomb is running down but the passengers are oblivious.) Which is a pretty useful perspective if you are plotting the course through your movie, knowing what each scene needs to mean for the storytelling reasons.
And just to be thorough, wiki says a scene is an action in a continous time. And a sequence is a series of scenes which form a distinct narrative unit, usually connected either by unity of location or unity of time. So acts built of sequences, sequences built of scenes, scenes built of shots (if you're thinking camera wise) or beats (if your thinking acting wise.)