[Menu and Menu buttons] Update website guidance to reflect complete parity

[alina-jacob]

Acceptance criteria

Menu buttons

• Update guidance under Menu buttons to show the new feature that allows basic and full/complex menus under all Menu buttons.

Menu (Modifiers)

• Update guidance under Menu to show the new feature that allows basic and full/complex menus under all Menu buttons.
• Include guidance around introducing decorative icons in a Menu. (Spec ref: Component proposal: Context menu carbon#6830 (comment) | PR ref: feat(menu): support menu modes carbon#14793)
• Introduce guidance around using shortcuts and nesting in a menu item.
• Introduce guidance around using icons and selection at the same time in a menu item.
• Update guidance around selection and multi-selection in a menu item. Decision to come from [Menu and Menu button - Multi selection] Menu's open / close status design research carbon#21246.
• [Menu buttons/Menu] Add the Context Menu variant #4354