docs: restructure documentation into audience-based portals

BREAKING CHANGE: Documentation URLs have changed

New structure:
- user/ - End-user documentation (device setup, sync guides, frontend)
- developer/ - Developer documentation (API reference, protocols, specs)
- operations/ - Operations documentation (deployment, troubleshooting)
- contributing/ - Contribution guides

Changes:
- Created portal INDEX.md files for each audience section
- Moved device guides to user/devices/ (kobo-setup.md, koreader-setup.md)
- Moved API docs to developer/ (api-reference.md, collections-api.md)
- Moved sync guide to user/sync-guide.md
- Moved troubleshooting to operations/troubleshooting.md
- Moved all split API docs to developer/api/
- Renamed protocol files (kobo-protocol.md, koreader-protocol.md)
- Added placeholder user guides (frontend, user-areas, settings, admin)
- Updated all internal links to new paths
- Updated Go code (http_handler.go, navigation.go) for new paths
- Updated main INDEX.md for audience-based navigation

Benefits:
- Clear separation of user and developer documentation
- Scalable structure for future user guide expansion
- Better organization and discoverability
- Audience-specific landing pages

Related to DOCS_IMPLEMENTATION_PLAN.md Phase 2 completion

internal/docs/http_handler.go

@@ -247,10 +247,11 @@ func (h *HTTPHandler) APIHome(c echo.Context) error {
 		TOC:     []templates.TOCItem{},
 		Breadcrumb: []templates.BreadcrumbItem{
 			{Title: "Docs", URL: "/docs"},
+			{Title: "Developer", URL: "/docs/developer"},
 			{Title: "API Reference", URL: ""},
 		},
 		Category:   "API Reference",
-		SourceFile: "API_REFERENCE.md",
+		SourceFile: "developer/api-reference.md",
 	}
 
 	var buf bytes.Buffer