docs: improve CLI help, shell completion, and README

- Add command groups (Getting Started, Search, Downloads, Daemon, System)
- Add shell completion command (bash, zsh, fish, powershell)
- Add flag completions for --type, --quality, --sort, --lang, --genre,
  --country, --method, --player
- Improve Long descriptions and Examples for all commands
- Split doctor disk check into platform-specific files (Unix/Windows)
- Validate infoHash length before truncating (prevent panic)
- Fix references to non-existent 'unarr daemon start' command
- Move stats command to System & Diagnostics group
- Rewrite README with complete documentation, correct config format
  (toml not yaml), all commands, shell completion section

internal/cmd/popular.go

@@ -20,8 +20,11 @@ func newPopularCmd() *cobra.Command {
 
 	cmd := &cobra.Command{
 		Use:   "popular",
-		Short: "Show popular content",
-		Long:  "Display the most popular movies and TV shows, ranked by community engagement.",
+		Short: "Show popular movies and TV shows",
+		Long: `Display the most popular movies and TV shows, ranked by community engagement.
+
+Results are ordered by trending score. Use --limit to control how many
+results to show and --page for pagination.`,
 		Example: `  unarr popular
   unarr popular --limit 20
   unarr popular --page 2 --json`,