feat(docs): comprehensive GitHub Primer theme overhaul with dark mode fix

- Fix broken dark mode by correcting palette toggle order and setting primary/accent to custom
- Rewrite extra.css with full GitHub Primer color scheme (light: #ffffff/#0969da, dark: #0d1117/#1f6feb)
- Style header, sidebar, buttons, cards, code blocks, and tables to match GitHub aesthetic
- Add Open Graph and Twitter Card meta tags via main.html override
- Add JSON-LD structured data (WebSite + SoftwareApplication schemas)
- Add announcement bar (v2.1.2 release)
- Add custom 404 page with navigation buttons
- Remove inline styles from skills/index.md (moved to extra.css)
- Mobile responsive adjustments for hero sections

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/overrides/404.html

@@ -0,0 +1,19 @@
+{% extends "main.html" %}
+
+{% block content %}
+<div style="text-align: center; padding: 4rem 1rem;">
+  <h1 style="font-size: 6rem; font-weight: 800; margin-bottom: 0.5rem; opacity: 0.15; line-height: 1;">404</h1>
+  <p style="font-size: 1.1rem; margin-bottom: 0.5rem; font-weight: 600;">
+    Page not found
+  </p>
+  <p style="font-size: 0.9rem; opacity: 0.6; margin-bottom: 2rem;">
+    The page you're looking for doesn't exist or has been moved.
+  </p>
+  <a href="{{ config.site_url }}" class="md-button md-button--primary">
+    Back to Home
+  </a>
+  <a href="{{ config.site_url }}skills/" class="md-button">
+    Browse Skills
+  </a>
+</div>
+{% endblock %}