Update ForwardDiff requirement to 1.3.1 in /docs (#195)

Updates the requirements on [ForwardDiff](https://github.com/JuliaDiff/ForwardDiff.jl) to permit the latest version.
- [Release notes](https://github.com/JuliaDiff/ForwardDiff.jl/releases)
- [Commits](JuliaDiff/ForwardDiff.jl@v0.0.2...v1.3.1)

---
updated-dependencies:
- dependency-name: ForwardDiff
  dependency-version: 1.3.1
  dependency-type: direct:production
...

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docs/Project.toml

@@ -16,4 +16,5 @@ Zygote = "e88e6eb3-aa80-5325-afca-941959d7151f"
 
 [compat]
 Documenter = "1.3"
+ForwardDiff = "1.3.1"
 Literate = "2.21.0"