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.

go.mod

@@ -19,7 +19,9 @@ require (
 )
 
 require (
+	github.com/alecthomas/chroma v0.10.0 // indirect
 	github.com/davecgh/go-spew v1.1.1 // indirect
+	github.com/dlclark/regexp2 v1.4.0 // indirect
 	github.com/gabriel-vasile/mimetype v1.4.12 // indirect
 	github.com/go-playground/locales v0.14.1 // indirect
 	github.com/go-playground/universal-translator v0.18.1 // indirect
@@ -35,6 +37,8 @@ require (
 	github.com/rogpeppe/go-internal v1.14.1 // indirect
 	github.com/valyala/bytebufferpool v1.0.0 // indirect
 	github.com/valyala/fasttemplate v1.2.2 // indirect
+	github.com/yuin/goldmark v1.7.16 // indirect
+	github.com/yuin/goldmark-highlighting v0.0.0-20220208100518-594be1970594 // indirect
 	golang.org/x/net v0.47.0 // indirect
 	golang.org/x/sync v0.19.0 // indirect
 	golang.org/x/sys v0.39.0 // indirect