docs: add signal forms examples to Checkbox and Chips

[msmallest]

Description

Part of this discussion: #32072 (comment)

Extra info

• Since the chips could have the same value, I converted the chips tracking function to refer to $index rather than identity. For just the new examples and the examples I derived these from.
• One example was based on the title "Chips with form control". So I named it "Chips with form field". While there is the Form Field Material primitive, I thought that the casing/spaces as well as the example still being about an actual FormField made it just fine in the end.