docs: document MQTT messages for Playnite Web Plugin

[cvele]

The Playnite Web Plugin utilizes MQTT for communication by publishing and subscribing to various topics. We need comprehensive documentation for these MQTT messages, with a particular focus on the topics that the plugin subscribes to. This documentation will help developers understand the communication patterns and facilitate troubleshooting and future development.

Tasks:

• List all MQTT topics that the Playnite Web Plugin subscribes to.
• Describe the purpose of each subscribed topic.
• Provide message structure for each topic, including expected payload format.
• Include examples of messages for each subscribed topic.
• Document any relevant QoS levels and their implications for message delivery.
• Outline the MQTT topics that the Playnite Web Plugin publishes to, including message structure and examples.
• Review existing code to ensure all MQTT messages are accounted for.
• Create a Markdown file in the repository (e.g., docs/mqtt-messages.md) to house this documentation.

Expected Outcome:

A comprehensive Markdown file documenting all MQTT topics the Playnite Web Plugin subscribes to and publishes, including their purposes, message structures, examples, and QoS levels.