Update AtomsIO requirement to 0.3.1 in /docs (#53)

Updates the requirements on [AtomsIO](https://github.com/mfherbst/AtomsIO.jl) to permit the latest version.
- [Release notes](https://github.com/mfherbst/AtomsIO.jl/releases)
- [Commits](https://github.com/mfherbst/AtomsIO.jl/commits)

---
updated-dependencies:
- dependency-name: AtomsIO
  dependency-version: 0.3.1
  dependency-type: direct:production
...

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docs/Project.toml

@@ -23,6 +23,7 @@ ASEconvert = "0.1.8, 0.2"
 AtomsBase = "0.5.2"
 AtomsBuilder = "0.2.3"
 AtomsCalculators = "0.2.3"
+AtomsIO = "0.3.1"
 DFTK = "0.7"
 Documenter = "~1.8, 1.16"
 EmpiricalPotentials = "0.2.5"