docs: Add 2-minute installation walkthrough video to Quick Start

[hashirshoaeb]

Summary

The Quick Start page (/getting-started) has a placeholder for a short video walkthrough of the installation process. This issue tracks creating and embedding that content.

Context

A {/* TODO: Add 2-minute installation walkthrough video */} comment was left in src/content/docs/getting-started/index.mdx during the Quick Start rewrite. The video would help visual learners get through installation quickly and reduce friction for first-time users.

Acceptance Criteria

• Record a ~2-minute screen recording covering the full installation flow (install CLI, authenticate, run shorebird init)
• Host the video (YouTube, Loom, or similar)
• Embed it in the Quick Start page, replacing the TODO comment
• Verify it renders correctly on the docs site

Notes

Keep the video short and focused, the goal is a quick visual confirmation of the steps already documented on the page, not a full tutorial.

Ref: #546