Improve ability to expose form fields from the API & add documentation

[saevarom]

Adding information about how to retrieve form fields from the API.

Please check the following:

• [x] Do the tests still pass?[^1]
• [x] Does the code comply with the style guide?
  • [x] Run make lint from the Wagtail root.
• [x] For Python changes: Have you added tests to cover the new/fixed behaviour?
• [x] For front-end changes: Did you test on all of Wagtail’s supported environments?[^2]
  • [x] Please list the exact browser and operating system versions you tested:
  • [x] Please list which assistive technologies [^3] you tested:
• [x] For new features: Has the documentation been updated accordingly?

Please describe additional details for testing this change.

[^1]: Development Testing [^2]: Browser and device support [^3]: Accessibility Target

[squash-labs[bot]]

Manage this branch in Squash

Test this branch here: https://saevaromapi-formfields-documen-lrvz7.squash.io

[lb-]

Note: I might be able to get this patched together from the WIP PR with tests https://github.com/wagtail/wagtail/pull/8782/files

I will flag myself as a reviewer and try to get this sorted.

[lb-]

I will need to take a look at the failing tests - looks like the data is getting duplicated in the tests.