[Doc] Document how to pass extra options to create-react-admin when using npm create

[slax57]

Problem

Documentation says we can use either npx or npm create with create-react-admin. However newcomers may get confused when passing additional arguments to the CLI, as both commands have a different syntax.

Solution

Document this gotcha and provide an example.

Additional Checks

• The PR targets master for a bugfix or a documentation fix, or next for a feature
• The documentation is up to date

Also, please make sure to read the contributing guidelines.