docs: Add CI/webhook integration guide for stack PRs

[Copilot]

pull_request webhook events now include a stack object for PRs that belong to a stack. Without documentation, teams can't easily discover how to use stack.base.ref to trigger CI on every PR in a stack targeting main — not just the bottom PR that directly targets main.

Changes

• New guide docs/src/content/docs/guides/ci-integration.md:

  • Documents the stack object shape in pull_request webhook payloads
  • Distinguishes pull_request.base.ref (direct PR base) from pull_request.stack.base.ref (ultimate stack target)
  • GitHub Actions if condition example covering both stacked and standalone PRs
  • JavaScript and Python webhook handler snippets with safe optional chaining for non-stacked PRs
  • Use cases for stack.base.sha (drift detection, targeted git log, CI caching)
  • Reference table mapping all four PR scenarios (standalone, bottom, mid-stack, top) to their field values

• docs/astro.config.mjs: Adds the new guide to the sidebar under Guides.

Key pattern documented

# GitHub Actions: run on any PR that ultimately targets main
if: |
  github.event.pull_request.base.ref == 'main' ||
  github.event.pull_request.stack.base.ref == 'main'

// Custom webhook handler — stack is absent for standalone PRs
const targetsMain =
  pr.base.ref === 'main' || pr.stack?.base?.ref === 'main';