Fix SPA heading hierarchy in generated workshop docs

[Copilot]

The workshop single-page renderer emitted every step title as an <h1>, producing 84 top-level headings in one document and breaking the accessibility heading structure. This change reserves <h1> for the overall workshop title and demotes per-step titles to section headings.

• Renderer output

  • Change each rendered workshop step title from <h1> to <h2> in scripts/build-docs.js
  • Keep the existing per-step structure and IDs intact so hash navigation and styling continue to work

• Document-level heading

  • Promote the persistent workshop title in the SPA header to the single document <h1>
  • Update the header markup/CSS to preserve the existing visual treatment with the new <h1><a> structure

• Regression coverage

  • Add a focused build-output test for the generated SPA to assert:
    • exactly one <h1>
    • no step titles rendered as <h1>
    • step titles rendered as <h2>

<header class="site-header">
  <h1 class="site-title">
    <a href="#00-welcome">GitHub Agentic Workflows Workshop</a>
  </h1>
</header>

<details id="07-your-first-workflow">
  <summary>Your first workflow</summary>
  <h2 id="07-your-first-workflow-title" class="workshop-page-title">
    Your first workflow
  </h2>
</details>

• Fixes [workshop-playwright-qa] __root__: 1 QA finding — accessibility #1801