docs(site): highlight augmenting existing ADO pipelines via stage generator

[Copilot]

Summary

Adds a mid-page section to the site landing page calling out that agents can augment existing Azure DevOps pipelines, positioned as a nice-to-have rather than a headline feature.

• New "Augment your existing pipelines" section in site/src/content/docs/index.mdx, placed between "Five security layers" and "Get started in minutes".
• Focuses on the target: stage generator: compile an agent into a reusable stage-level template that drops into an existing multi-stage pipeline, with a short YAML snippet and a link to the Target platforms reference.

stages:
  - stage: Build
    jobs: ...
  - template: agents/review.lock.yml   # the agentic workflow, slotted in
    parameters:
      dependsOn: Build

Test plan

Documentation-only .mdx change. Snippet and target: stage semantics verified against docs/targets.md; internal link path matches existing /ado-aw/reference/targets/ route.