Document convenient syntax of fj operations

[hannobraun]

The fj crate defines various operations, like fj::Sweep or fj::Transform. Those are just Rust structs, and they can be combined with each other to form more complex operations.

This can become very tedious, so the fj crate also defines a more convenient syntax via extension traits in the syntax module. For example, fj::Sweep::from_path(shape, path) can be replaced with shape.sweep(path). This can make a real difference with more complex operations, as the extension trait methods can be chained.

The problem is, that the convenient syntax is not easily discoverable in the API reference, unless the reader happens to come across the syntax module (and knows how to understand it). The documentation of each struct that has convenient syntax available should have an # Examples section, which provides examples and documents the convenient syntax.

I'm labeling this as https://github.com/hannobraun/Fornjot/labels/good%20first%20issue, since not a lot of insight into Fornjot should be required to address this. Anyone with sufficient Rust experience (specifically experience with extension traits) should be able to figure out the convenient syntax and write documentation about it. In addition, the models in the models/ directory use the convenient syntax, so examples are available.