feat(docs): create sukr documentation site

Self-documenting docs site built with sukr itself (dogfooding):
- docs/site.toml with sukr.io base URL
- docs-specific templates with sidebar navigation
- Dark theme CSS, responsive layout
- Documentation: getting-started, configuration, features

Also: improved error chaining for better template debugging

docs/templates/content/default.html

@@ -0,0 +1,9 @@
+{% extends "base.html" %} {% block content %}
+<article class="doc-page">
+  <h1>{{ page.title }}</h1>
+  {% if page.description %}
+  <p class="lead">{{ page.description }}</p>
+  {% endif %}
+  <section class="content">{{ content | safe }}</section>
+</article>
+{% endblock content %}