Docs: Add tutorial for writing custom controllers

[amytsengtw]

Summary

This PR adds a new tutorial document explaining how to implement custom controller modules for non-Android devices (e.g., smart lights, PDUs). It includes a complete example with type hinting and proper error handling.

Solves

Closes #613

Changes

• Added docs/tutorial_custom_controller.md with a SmartLight example.
• Updated docs/index.rst to include the new tutorial in the table of contents.

Verification

I have verified that the example code adheres to Google Python style guidelines and passes local checks.