chore(dx): add terraform-docs install to setup scripts and docs

[biefan]

Summary

• add terraform-docs v0.21.0 installation to setup-dev.sh with SHA256 verification against release checksums
• add the same pinned installation and checksum verification flow to setup-dev.ps1
• skip installation when the correct terraform-docs version is already present
• document terraform-docs v0.21.0 in docs/contributing/prerequisites.md and add local regeneration command guidance

Testing

• bash -n setup-dev.sh
• npm run lint:md
• npm run spell-check
• Not run: PSScriptAnalyzer (pwsh is not available in this environment)

Resolves #199

[WilliamBerryiii]

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