Add example for collapsing fields using custom FieldTemplate

[jglev]

Reasons for making this change

The current documentation contains a call for an example of collapsing fields in a form. This PR adds such an example, using a custom FieldTemplate.

This relates to #268, and hopefully resolves #598 -- the example I've linked doesn't show it, but I currently have also implemented an "Expand / Hide All" button using the React state of the component housing the <Form> component and the formContext object that's used in the linked example.

Checklist

• I'm updating documentation
  • I've checked the rendering of the Markdown text I've added
• I'm adding or updating code
  • I've added and/or updated tests
  • I've updated docs if needed
  • I've run npm run cs-format on my branch to conform my code to prettier coding style
• I'm adding a new feature
  • I've updated the playground with an example use of the feature

[epicfaace]

Thanks! Looks good. In the future, it might be nice to add this behavior into the library itself by adding an attribute collapsible under ui:options.

[jglev]

Thank you very much!

In the future, it might be nice to add this behavior into the library itself by adding an attribute collapsible under ui:options.

That sounds good to me, too! I may do some work on that if/when I have the time in the next few weeks.