Document the "plugin" decorator

[sambarza]

Overwritable methods:

• settings_schema (check if method must be documented, settings_model is better)
• settings_model
• load_settings
• save_settings

[kodaline]

@sambarza we added those in the Hooks table here [1], is it enough to close this issue? Or do you have something more in mind?

[1] https://cheshire-cat-ai.github.io/docs/technical/plugins/hooks/#__tabbed_1_4