Update README.md and versioning information

[integr-dev]

This pull request makes significant improvements to the documentation and configuration of the Backbone plugin, focusing on expanding the README with detailed usage instructions, examples for new features, and updating the plugin metadata. The changes provide clearer guidance for users on installation, requirements, available commands, and advanced features like custom items, entities, and display rendering.

Documentation Enhancements:

• Expanded the README.md to include detailed sections on requirements, installation, and management commands (/bb scripting, /bb item, /bb entity), making it easier for users to get started and manage the plugin.
• Added comprehensive usage examples for new frameworks: custom items, custom entities, and display entity rendering, including code samples and lifecycle integration.
• Improved and clarified configuration and command registration examples, updating method names and usage to reflect the latest API (e.g., Backbone.Handlers.COMMAND.register, configHandler.writeState). [1] [2]
• Updated GUI framework documentation to use new lifecycle method names (onOpen, onClose, etc.) and added a section on PlaceholderAPI integration.

Plugin Configuration Updates:

• Updated plugin.yml to use a variable for the version, improved the description, and listed new features such as custom items, entities, and display entity rendering.

Removed Redundant/Outdated Sections:

• Removed the old "Managing Scripts" section in favor of the new, more detailed management commands documentation.