Generate syntax/railroad diagram

[ronanM]

Please complete the following tasks

• [X] I have searched the discussions
• [X] I have searched the open and rejected issues

Clap Version

master

Describe your use case

For documentation purpose, add generation of syntax diagram:

• https://en.wikipedia.org/wiki/Syntax_diagram

Describe the solution you'd like

Maybe using https://github.com/lukaslueg/railroad

Alternatives, if applicable

No response

Additional Context

No response