docs: add interactive documentation system with Go+HTMX

- Add internal/docs package with markdown renderer (goldmark)
- Create docs layout template with sidebar navigation
- Implement hierarchical navigation auto-generated from docs folder
- Add table of contents generator (extract ## headings)
- Add syntax highlighting for code blocks (highlight.js)
- Add mobile responsive design
- Add /docs routes to main.go

The documentation system features:
- Dark theme matching app design
- Collapsible sidebar sections (Getting Started, User Guide, Device Setup, API Reference, Contributing)
- Table of contents for each page
- Breadcrumb navigation
- Full-text search (client-side JavaScript, API endpoint ready)
- Syntax highlighting for code blocks
- Mobile-friendly with hamburger menu

All documentation is served from /docs route, no authentication required.
Markdown files are rendered using goldmark with GFM extensions and syntax highlighting.

templates/types.go

@@ -123,3 +123,40 @@ type PotentialMatchData struct {
 	MatchMethod    string
 	Confidence     float64
 }
+
+// Documentation types
+type Navigation struct {
+	Sections []NavSection
+}
+
+type NavSection struct {
+	Title     string
+	Items     []NavItem
+	Collapsed bool
+}
+
+type NavItem struct {
+	Title string
+	URL   string
+	Icon  string
+}
+
+type Document struct {
+	Title      string
+	Content    string
+	TOC        []TOCItem
+	Breadcrumb []BreadcrumbItem
+	Category   string
+	SourceFile string
+}
+
+type TOCItem struct {
+	Level  int
+	Title  string
+	Anchor string
+}
+
+type BreadcrumbItem struct {
+	Title string
+	URL   string
+}