docs: document packages in release evidence

Tim Rizzirequested to merge
trizzi/docs-release-evidence-packages into master

What does this MR do and why?

Adds documentation for the automatic inclusion of packages in release evidence, introduced in !227172 (merged).

Changes

  • Updates the introductory paragraph to mention packages alongside milestones and test artifacts
  • Adds a packages entry to the example release evidence JSON
  • Adds a new Include packages as release evidence section explaining:
    • How version matching works (including v/V prefix stripping)
    • Which packages are included (displayable only)
    • The fields exposed for each package (id, name, version, package_type, created_at)

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