docs: use correct import paths in examples

[SAY-5]

The documentation examples import Application and Command from the top-level cleo package (from cleo import Application, from cleo import Command), but cleo/__init__.py only exposes __version__, so these examples raise ImportError when run.

This updates the examples in introduction.rst, usage.rst, and single_command_tool.rst to use the canonical paths already used in the README and tests:

• from cleo.application import Application
• from cleo.commands.command import Command