[Bug]: Improve Extension Documentation for catalog

[dhilipkumars]

Bug Description

Extension Documentation should now include these

specify extension catalog --help
                                                                                    
 Usage: specify extension catalog [OPTIONS] COMMAND [ARGS]...                       
                                                                                    
 Manage extension catalogs                                                          
                                                                                    
╭─ Options ────────────────────────────────────────────────────────────────────────╮
│ --help          Show this message and exit.                                      │
╰──────────────────────────────────────────────────────────────────────────────────╯
╭─ Commands ───────────────────────────────────────────────────────────────────────╮
│ list     List all active extension catalogs.                                     │
│ add      Add a catalog to .specify/extension-catalogs.yml.                       │
│ remove   Remove a catalog from .specify/extension-catalogs.yml.                  │
╰──────────────────────────────────────────────────────────────────────────────────╯

Steps to Reproduce

Update the doc: https://github.com/github/spec-kit/blob/main/extensions/EXTENSION-USER-GUIDE.md

Expected Behavior

n/A

Actual Behavior

n/A

Specify CLI Version

0.2.2

AI Agent

Antigravity

Operating System

macos

Python Version

Python 3.9.6

Error Logs

N/A

Additional Context

No response