content: React/Vue Quickstart guides use legacy form control syntax

[averyjohnston]

URL

https://ionicframework.com/docs/react/quickstart, https://ionicframework.com/docs/vue/quickstart

Issue Description

Under "A component with style," both pages have code snippets that use the legacy syntax for ion-checkbox (see checkbox docs page for more info).

The surrounding content in the section also references the syntax and will need updating as well; examples:

• The checkbox is currently slotted in an ion-item, which is used to explain the slot property. The explanation may need to be changed to refer to the ion-badge instead.
• On the React page, the checkbox is referred to as not having any children, but this will change if the label text is moved into the default slot.

There may be other nearby things that need tweaking. The entire section should be reviewed to ensure everything still makes sense.