DOC: add SIL parachute ejection integration example (#524)

[thatrandomasiandev]

Summary

• Add docs/user/sil_parachute_ejection.rst showing Software-in-the-Loop parachute ejection detection wired through Flight / parachute trigger callables.
• Link the page from the user guide toctree and from the parachute triggers "See Also" section.
• Include a simplified stateful barometric detector, dual-deploy attachment pattern, notes on wrapping compiled recovery firmware, and a cautious HIL mention; points to the Valetudo Monte Carlo inspiration.

Fixes #524

Test plan

• Confirm toctree entry renders under Special Case Simulations
• Skim sil_parachute_ejection.rst for Sphinx link resolution (:doc:, :ref:, :class:)
• Optional: cd docs && make html if the docs env is available