Planette/Forms: added article about ->toggle()

There is currently no mention of `toggle()` in Forms documentation, but the page is quite long, so I rather wrote Planette article. I can try to translate at some point, but if anybody is willing, you are welcome :). Also it would be great if someone reviews the content if there is no toggle-related feature or best-practice missing.

- [x] is this best practice?
- [ ] EN translation
- [x] all features documented