UCP: support help command

[you06]

Description

• Move all available commands into a single package, and add descriptions for them.
• /help command will comment with basic usage comment.

Bonus

/help command comments different guide for different people, consider the access.

Difficulty

• Easy

Score

• 300

The extra bonus worth 200 points.

Mentor

@you06

Recommended Skills

• Go

[sunl888]

Are all the available commands in cmd/bot.go？

[you06]

@wq1019 You only need to consider the commands in this bot.

To find all commands, you can look into the issue comment handle function in providers to find all the available commands.
https://github.com/pingcap-incubator/cherry-bot/blob/70abd98e8864e5b80bc9cb6ad1497512339724bd/bot/webhook.go#L71-L97

[you06]

Maybe you can collect all the commands into pkg/types or a new package.