doc (Foldable): Add examples to Data.Foldable

• are either individually buildable or squashed
• have commit messages which describe what they do (referring to Notes and tickets using #NNNN syntax when appropriate)
• have added source comments describing your change. For larger changes you likely should add a Note and cross-reference it from the relevant places.
• add a testcase to the testsuite.
• Add examples to Data.Foldable

Tracking: #17929

@core-libraries