Add usage guidance for select vs combobox vs radio

[jattasNI]

Pull Request

🤨 Rationale

I needed an example of component usage guidance for an NITech presentation

👩‍💻 Implementation

Capture my understanding of when to use select, combobox, and radio in each of their MDX files.

I moved the combobox description from storybook to mdx because it was annoying to format large markdown strings from code.

Updated language for existing usage guidance to match new pattern.

• my secret motivation is working around a prettier bug that puts tags near the end of a line in a new paragraph, leading to ugly docs. I couldn't find a syntax to prettier-ignore in mdx files. nor can other people

🧪 Testing

Local storybook

✅ Checklist

• I have updated the project documentation to reflect my changes or determined no changes are needed.

[rajsite]

@fredvisser straightforward change to docs that I don't think are controversial. Bypassing but if concerns can address in follow-up.