content: document focusable classes and usage

### Prerequisites

- [X] I have read the [Contributing Guidelines](https://github.com/ionic-team/ionic-framework/blob/main/.github/CONTRIBUTING.md#creating-an-issue).
- [X] I agree to follow the [Code of Conduct](https://ionicframework.com/code-of-conduct).
- [X] I have searched for [existing issues](https://github.com/ionic-team/ionic-framework/issues) that already report this problem, without success.

### Ionic Framework Version

v6.x, v7.x

### Current Behavior

.ion-focused nor :focus-visible selector is working for ion-toggle component and toggle/labels combo.

There are workarounds for this but not optimal
- use ion-item, but ion-item is expected to be used under restricted scenarios
- use :focus selector but causes the style for accessibility leak when non-keyboard focus is applied.

### Expected Behavior

- ion-toggle should provide a default keyboard focus accessibility style.
- ion-toggle should expose a way to set custom focus accessibility style

### Steps to Reproduce

1. go to https://stackblitz.com/edit/ionic-ion-toggle-focus-isse
2. use keyboard to navigate.
3. background of focus-visible element SHOULD BE RED, similar to the button

### Code Reproduction URL

https://stackblitz.com/edit/ionic-ion-toggle-focus-isse

### Ionic Info

Ionic:

Ionic CLI : 7.1.1
Ionic Framework : @ionic/angular 6.2.7
@angular-devkit/build-angular : 17.1.1
@angular-devkit/schematics : 17.1.1
@angular/cli : 17.1.1
@ionic/angular-toolkit : 7.0.0

Capacitor:

Capacitor CLI : 5.2.3
@capacitor/android : 5.2.3
@capacitor/core : 5.2.3
@capacitor/ios : 5.2.3

Utility:

cordova-res : not installed globally
native-run : 1.7.4

System:

NodeJS : v20.11.0 (C:\Program Files\nodejs\node.exe)
npm : 10.2.4
OS : Windows 10

### Additional Information

_No response_