docs: add `chatgpt-cli` to CLI Tools section

[efJerryYang]

This repo provides a list of the most frequently used commands to simulate the experience of using the official ChatGPT web client, including:

• !help: shows the help message
• !show: displays the current conversation messages
• !save: saves the current conversation to a JSON file
• !load: loads a conversation from a JSON file
• !new or !reset: starts a new conversation
• !regen: regenerates the last response
• !resend: resends your last prompt to generate response
• !edit: selects messages for editing
• !drop: selects messages for deletion
• !exit or !quit: exits the program

Support markdown rendering for both user and assistant's output.

Conversations are saved locally in JSON format, can be loaded or saved by using commands !load and !save.

Users will be prompted to configure their OpenAI API key when they run the program for the first time. They can also configure it manually and edit the config.yaml file. A template is provided in the repo.