feat(doc): generate documentation for UDVTs

[PaulRBerg]

Component

Forge

Describe the feature you would like

forge doc doesn't generate any documentation for user-defined value-types.

Ideally, forge doc would pick up all functions associated with a UDVT, and aggregate their NatSpec in a file type.Name.sol.

Example type: UD60x18 part of PRBMath.

Additional context

No response