feat(docs): real FAQ accordion + redirect at /docs/index.html

FAQ:
- Restructures faq.mdx to wrap each Q/A in a native <details>/<summary>
  element with class="faq-item". The previous version was just bold h6
  questions followed by paragraphs -- which read as a wall of text. Now
  each question is a click-to-expand row.
- Native HTML disclosure -- no JS required, keyboard-accessible by
  default (Enter/Space to toggle, focusable summary).
- Adds .faq-item styling to _docs.scss: rounded card-like items with
  a hover border-color shift, primary-tinted border + subtle shadow
  when open, chevron icon on the right that rotates 90deg on open,
  primary-color summary text when open. Open content padded under the
  summary with a hairline divider.

/docs/ landing:
- Astro with build.format="file" builds pages/docs/index.astro to
  dist/docs.html, NOT dist/docs/index.html -- which meant /docs/ and
  /docs/index.html returned 404/403 on prod. Updates the flatten-build
  npm script to move dist/docs.html -> dist/docs/index.html as the
  final step. Both URLs now serve the meta-refresh redirect to
  introduction.html.

Bundlewatch:
- Bumps CSS budgets +2KB (46->48 unminified, 42->44 minified) to
  accommodate the FAQ accordion + docs-prose typography upgrades.