Add a doc section on composables with guidelines about how to write them in the correct idiomatic way

There are some significant composable libraries out there. And, since using the composition api and composables is such a great reuse tool it would be nice to know how to avoid problems.

Document the "useFeature" paradigm and how it interacts with the component scope and how reactivity works.
Examples, some of which I still don't understand fully:

• why the useFeature naming is a good idea
• does useFeature need to be exported by name like import {useRoute} from 'vue-router or should I export it as the default
• global vs local/component scope in the composable
• if I create a composable useFeature and it needs to use vue router then useRouter() must be inside the useFeature function so it ends up in the proper scope in the component.
• how does reactivity work
  • computed function inside useFeature function
  • computed function outside useFeature function
  • passing a component prop to useFeature(props.something)
  • etc

Start by reviewing the existing Vue documentation structure and current guidance on the Composition API and composables. Cover the useFeature naming and export questions, component scope, vue-router usage, and the listed reactivity cases; done when the guidance and examples explain each question clearly.