### What?
Adds comprehensive documentation for the RadioField component in the
form builder plugin documentation.
### Why?
Addresses review feedback from #11908 noting that the RadioField was not
documented after being exported for end-user use.
### How?
- Added RadioField section with complete property documentation
- Added detailed Radio Options sub-section explaining option structure
- Updated Select field documentation for consistency (added missing
placeholder property and detailed options structure)
- Added radio field to configuration example to show all available
fields
Fixes the documentation gap identified in #11908 review comments.
4e972c3fe2