docs: add feature showcase examples

[frydj]

Summary

Adds 7 feature-focused example datasets to Examples > Feature Showcases:

Policy & Access Control

• Users, organizations, documents with policies
• Demonstrates f:Policy rules and testing

Reasoning & Inference

• Biological taxonomy with OWL rules
• Demonstrates transitive/symmetric properties

Full-Text Search

• Recipe database with search
• Demonstrates Lucene integration

SHACL Validation

• Shapes and validation examples
• Demonstrates constraint violations

Time-Travel & Audit

• Data with historical queries
• Demonstrates as-of and history queries

SQL Migration

• Side-by-side SQL/FlureeQL comparison
• Helps SQL users transition

REST API Patterns

• CRUD operations via HTTP
• Demonstrates API integration

Each includes seed data JSON files and tutorial guides.

Test plan

• Load seed data into test instance
• Verify feature examples demonstrate advertised capabilities

[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 👈 (View in Graphite)
• 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
• 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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