Add JVP functions to API documentation

- Added finite_difference_jvp and finite_difference_jvp! to docs
- Added JVPCache documentation section
- Included explanation of JVP functionality and use cases
- Documentation now builds cleanly without warnings

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: ChrisRackauckas

docs/src/api.md

@@ -56,3 +56,14 @@ FiniteDiff.HessianCache
 ```
 
 Hessians are for functions `f(x)` which return a scalar.
+
+## Jacobian-Vector Products (JVP)
+
+```@docs
+FiniteDiff.finite_difference_jvp
+FiniteDiff.finite_difference_jvp!
+FiniteDiff.JVPCache
+```
+
+JVP functions compute the Jacobian-vector product `J(x) * v` efficiently without computing the full Jacobian matrix. This is particularly useful when you only need directional derivatives.
+