[AbuMareBear]

Summary

• Bump bridgetown-stoa from 0.1.0 to 0.4.0 in the docs site (conservative bump; no other gems changed).
• Wire in the theme's styles, which since 0.2.0 ship as a Tailwind v4 partial via the companion bridgetown-stoa npm package: install tailwindcss, @tailwindcss/postcss and bridgetown-stoa, and import them from frontend/styles/index.css.
• Replace the default PostCSS plugins (postcss-preset-env, postcss-flexbugs-fixes) with @tailwindcss/postcss, matching Stoa's own fixture site. Tailwind v4 handles prefixing itself.
• Adopt the 0.4.0 sidebar: give the home page a title and nav_order, and mark the 404/500 pages nav_exclude. Rule pages are listed top-level, sorted by title.

Test plan

• bundle exec bridgetown deploy builds cleanly with frontend bundling
• Sidebar renders Home plus both rule pages; 404/500 excluded
• Compiled CSS contains the Stoa layout and content styles
• Check the deployed site at https://a11y-lint.guidedrails.com after merge