#[SettingsGroup]
Attribute on classes.
Declares a group of settings of a theme, child theme or plugin.
The class is readonly and declares each setting as a public promoted
constructor parameter with a default, like a block attribute class: the
types and markers of values (#[ListOf], #[Url], #[Label], #[Hidden],
#[Image] for an attachment ID) derive the schema and the form. The framework stores the group in the option
{namespace}_{name} (namespace of the theme or plugin with "_" instead of "-"), edits it on the settings page
of the theme or plugin and injects it into the constructor of services that
declare a parameter of its type; templates read it as settings.{name}.