feat(docs): create sukr documentation site with fixes

Self-documenting docs site built with sukr itself (dogfooding):

Core changes:
- Rename package from nrd-sh to sukr
- Move personal site to sites/nrd.sh/
- Update AGENTS.md and README.md

Documentation site (docs/):
- Add site.toml with sukr.io base URL
- Create docs-specific templates with sidebar navigation
- Add dark theme CSS with syntax highlighting colors
- Document all features: templates, sections, syntax highlighting,
  mermaid diagrams, and LaTeX math rendering

Bug fixes:
- Render individual pages for all sections (not just blog type)
- Add #[source] error chaining for Tera template errors
- Print full error chain in main() for better debugging

docs/content/features/mermaid.md

@@ -0,0 +1,70 @@
+---
+title: Mermaid Diagrams
+description: Build-time diagram rendering with Mermaid
+weight: 4
+---
+
+# Mermaid Diagrams
+
+sukr renders Mermaid diagrams at build time, producing inline SVG. No client-side JavaScript required.
+
+## Usage
+
+Use fenced code blocks with `mermaid` language:
+
+````markdown
+```mermaid
+graph LR
+    A[Input] --> B[Process]
+    B --> C[Output]
+```
+````
+
+## Supported Diagram Types
+
+| Type      | Status          | Description                               |
+| --------- | --------------- | ----------------------------------------- |
+| Flowchart | ✅ Full support | Flow diagrams with nodes and edges        |
+| Graph     | ✅ Full support | Directed/undirected graphs                |
+| Sequence  | ⚠️ Partial      | Sequence diagrams (some layouts may vary) |
+| State     | ⚠️ Partial      | State machine diagrams                    |
+| Class     | ⚠️ Partial      | UML class diagrams                        |
+
+> **Note:** Some complex diagram types may have layout differences compared to the JavaScript renderer. Flowcharts and basic graphs work reliably.
+
+## How It Works
+
+1. During Markdown parsing, `mermaid` code blocks are intercepted
+2. The Mermaid definition is parsed and rendered to SVG
+3. The SVG is inlined directly in the HTML output
+4. No JavaScript or external resources needed
+
+## Example
+
+```mermaid
+graph TD
+    A[Markdown] --> B{sukr}
+    B --> C[HTML]
+    B --> D[SVG Diagrams]
+    B --> E[Highlighted Code]
+```
+
+## Styling
+
+Mermaid SVGs inherit your CSS variables. Customize the look by targeting SVG elements:
+
+```css
+.mermaid svg {
+  max-width: 100%;
+  height: auto;
+}
+
+.mermaid .node rect {
+  fill: var(--bg-sidebar);
+  stroke: var(--accent);
+}
+```
+
+## Fallback
+
+If a diagram fails to render (complex diagrams, syntax errors), the original code block is preserved with an error comment.