docs: add security guides

[frydj]

Summary

New Learn > Security & Access Control section with 3 guides:

• data-access-control-concepts.mdx: Policy-based security model overview
• writing-policies.mdx: Creating f:Policy rules for data access
• verifiable-data.mdx: (moved from foundations/) Cryptographic signing and verification

Removed

• guides/policy-old.mdx: Deprecated content

Test plan

• Verify policy examples match current API
• Check security concepts align with reference docs

[frydj]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• docs: add feature showcase examples #71
• docs: add domain example datasets #70
• docs: add feature reference docs #69
• docs: reorganize cookbook #68
• docs: restructure querying reference #67
• docs: restructure HTTP API reference #66
• docs: add advanced topics #65
• docs: add security guides #64 👈 (View in Graphite)
• docs: add modeling guides #63
• docs: add working with data guides #62
• docs: reorganize learn foundations #61
• docs: add getting started guides #60
• docs: add project config & infrastructure #59
• main

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