docs: Content of user_guide.md not published at cobra.dev

[mloskot]

This PR long ago

• readme: split 'Getting Started' into 'user_guide.md' #1431

extracted parts of the README.md into new user_guide.md, and offered links in the former to content in the latter.

The current README.md contains this

For complete details on using the Cobra library, please read the The Cobra User Guide.

However, AFAICT, content published at https://cobra.dev do not include (lost?) any references to the user_guide.md, so the website does not lead a reader to the (still relevant?) content of the user guide and its precious section on Organizing subcommands which could address number of SO and Reddit questions 😊