docs: add variable-groups field to README table and site front-matter reference

[github-actions]

Summary

• The variable-groups: front-matter field (added in feat(compile): support Azure DevOps variable group imports #1426) was completely absent from the human-facing docs. Users evaluating or setting up a pipeline that needs ADO Library secrets had no docs-site or README coverage to guide them.

Changes

• README.md — new variable-groups row in the Front Matter Fields table (after repos:)
• site/src/content/docs/reference/front-matter.mdx — two additions:
  • Overview YAML block: variable-groups: commented entry after repos:
  • New ## Variable Groups section covering:
    • Basic usage with variables: - group: output shape
    • The ADO two-step requirement (authorize + import)
    • Names-only rule (secrets never resolved or logged)
    • Practical example: wiring a GitHub App private key from a group
    • Validation rules (injection, whitespace, duplicates)
    • Target support (standalone/1es only; compile error on job/stage)

Accuracy checks

• variable_groups: Vec<String> field confirmed in src/compile/types.rs (line 1109)
• Validation rules confirmed in src/compile/common.rs (validate_variable_groups, lines 451–513)
• Target restriction (Job/Stage rejected) confirmed in same function
• PipelineVar::Group lowering confirmed in src/compile/common.rs::variable_group_vars

Validation

• cd site && npm run build:astro — 38 pages built, all internal links valid
• README re-read for valid Markdown and working links

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