Simplify the quickstart and make it more imperative

[djsauble]

Preview: https://controlflow-make_the_quickstart_more_linear_and_imperative.mintlify.app/quickstart

• Simplify the explanations of key concepts in the quickstart, and cross-link to the full concept pages for people who want the more detailed explanation.
• Move the explanation of code before the code block it describes to more clearly set expectations.
• Rename section titles to be action-focused ("do the thing") instead of purely descriptive ("the thing").
• Convert paragraphs to ventilated prose so that future edits are easier to review.
• Remove explicit --- line separators since the headings introduce enough negative space on their own.
• Code styling suggestion: shorten code examples by merging back-to-back empty lines.

[jlowin]

Thanks!