Support version pinning for terraform-docs in the Terraform feature

[Copilot]

terraform-docs could only be toggled on/off via installTerraformDocs, always installing latest. This left builds exposed to breaking changes across releases (e.g. v0.22.0's altered table delimiter formatting). This adds a version pin.

Changes

• New option terraformDocsVersion (string, default latest) in devcontainer-feature.json. Resolution flows through the existing find_version_from_git_tags, so latest and specific versions both work.
• install.sh: reads TERRAFORMDOCSVERSION into TERRAFORM_DOCS_VERSION, replacing the hardcoded latest. The value is only consumed inside the installTerraformDocs block, so it has no effect unless that flag is set.
• Tests: added install_terraform_docs_version scenario + test asserting a pinned 0.20.0 install.
• Version bump: 1.4.5 → 1.5.0; README options table updated.

Usage

"features": {
    "ghcr.io/devcontainers/features/terraform:1": {
        "installTerraformDocs": true,
        "terraformDocsVersion": "0.20.0"
    }
}

• Fixes Support version pinning for terraform-docs in the Terraform feature #1630