### What?
Updated the existing Virtual Fields section to focus on Join field types
and added a new "Virtual Field Configuration" section covering how to
make any field virtual.
### Why?
The existing virtual fields documentation was lacking depth and clarity.
### How?
- Refined existing h3 "Virtual Fields" section to focus specifically on
Join field types
- Added new h2 "Virtual Field Configuration" section with detailed
coverage of:
- Boolean virtual fields with hooks for computed values
- String path virtual fields for relationship resolution
- Virtual path syntax and requirements
- Common use cases with practical examples
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