docs: add reading order for the security model to GETTING_STARTED.md

[AaliyaKhan25]

Summary

Checklist

• The issue this closes was assigned to me (see CONTRIBUTING.md)
• This is my only open PR (one issue at a time, see CONTRIBUTING.md)
• Target branch is dev (see BRANCHING.md)
• cd sdk && make check-ci passes locally
• New code has tests (every module gets a test file)
• Public API changes are reflected in sdk/README.md / sdk/docs/
• No secrets, internal URLs, or private paths in the diff
• If a model wrote a meaningful part of this, I said so below (AI_POLICY.md)

Notes for reviewers

[greptile-apps]

No reviewable files after applying ignore patterns.

[AaliyaKhan25]

Closing in favor of a cleaner branch — this one had picked up an unrelated commit from another PR (guard.py). Reopening from a fresh branch off dev.