Update Literate requirement to 2.21.0 in /docs (#194)

Updates the requirements on [Literate](https://github.com/fredrikekre/Literate.jl) to permit the latest version.
- [Release notes](https://github.com/fredrikekre/Literate.jl/releases)
- [Changelog](https://github.com/fredrikekre/Literate.jl/blob/master/CHANGELOG.md)
- [Commits](fredrikekre/Literate.jl@v0.1.0...v2.21.0)

---
updated-dependencies:
- dependency-name: Literate
  dependency-version: 2.21.0
  dependency-type: direct:production
...

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docs/Project.toml

@@ -16,3 +16,4 @@ Zygote = "e88e6eb3-aa80-5325-afca-941959d7151f"
 
 [compat]
 Documenter = "1.3"
+Literate = "2.21.0"