docs: adopt Avm.Authoring contribution workflow

[jaredfholgate]

Summary

• replace the legacy Docker-based ./avm workflow with the Avm.Authoring PowerShell module
• document managed tools and local unit, integration, and e2e test commands
• warn module owners to update branch-protection checks for per-example e2e jobs
• update Terraform prerequisites and TFNFR5 to use the new command surface

Validation

• markdownlint on changed files
• Hugo build with CI-pinned Hugo 0.136.5
• confirmed no legacy ./avm, tf-test-unit, or tf-test-integration commands remain under docs/