Simplify quickstart page

[mintlify]

Condensed the quickstart guide by removing verbose explanations and detailed step-by-step instructions. The simplified version focuses on essential actions: deploying the site and making the first change.

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Note

Low Risk
Documentation-only changes with no runtime or API impact; risk is limited to potentially omitting guidance users previously relied on.

Overview
Condenses quickstart.mdx to a minimal Quickstart focused on two actions: deploying via mintlify.com/start and making an initial edit to index.mdx (web editor or CLI).

Removes the detailed onboarding explanation, optional GitHub-skip accordion, dashboard screenshots, and the step-by-step CLI/Web Editor walkthrough, replacing them with short instructions, a tiny mint dev snippet, and a single publish/deploy note.

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