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/sections.md

@@ -12,7 +12,7 @@ sukr automatically discovers sections from your content directory structure.
 
 A section is any directory under `content/` that contains an `_index.md` file:
 
-```
+```text
 content/
 ├── _index.md           # Homepage (not a section)
 ├── about.md            # Standalone page