Add Quick Start text fallback for the demo video

[Copilot]

The Quick Start page relied on an embedded demo video for its strongest onboarding path, which left restricted environments with only the browser’s HTML5 video fallback message. This adds a readable, collapsible text alternative directly under the demo so the page remains usable without video playback.

• Quick Start fallback summary

  • Added a <details> block immediately below the Quick Start demo video in docs/src/content/docs/setup/quick-start.mdx
  • Captures the core flow shown in the video:
    1. install gh-aw
    2. run gh aw add-wizard
    3. choose an engine and configure the matching secret
    4. add the workflow and compiled lock file
    5. watch the run and check the generated issue

• Accessible structure

  • Used a native collapsible <details> / <summary> pattern
  • Rendered the walkthrough as an ordered list so the sequence is explicit in both visual and assistive-tech contexts

• Focused docs coverage

  • Added a dedicated Playwright spec for the Quick Start page
  • Verifies the summary exists, starts collapsed, and includes the documented steps

<details>
  <summary>Prefer text? Read the Quick Start demo summary</summary>
  <ol>
    <li>Install the extension with <code>gh extension install github/gh-aw</code>.</li>
    <li>From your repository root, run <code>gh aw add-wizard githubnext/agentics/daily-repo-status</code>.</li>
    <li>Choose an AI engine and set the matching repository secret when prompted.</li>
  </ol>
</details>