docs: improve azurerm_firewall_policy example usage

[yosofbadr]

Summary

• Adds a comprehensive "Complete Example" to the azurerm_firewall_policy documentation showing all optional blocks with Premium SKU
• Includes working examples of: dns, identity, insights (with log_analytics_workspace), intrusion_detection (with signature_overrides, traffic_bypass, and private_ranges), threat_intelligence_allowlist, tls_certificate, explicit_proxy, private_ip_ranges, auto_learn_private_ranges_enabled, sql_redirect_allowed, and tags
• Retains the existing minimal example for basic usage

Fixes #15585

Test plan

• Verify the HCL example is syntactically valid
• Confirm all optional blocks documented in the Arguments Reference are represented in the new example
• Review that supporting resources (Key Vault, User Assigned Identity, IP Groups, Log Analytics Workspace) are correctly defined

[sreallymatt]

Hi @yosofbadr - as mentioned in #31006, we generally don't add multiple large examples in the documentation. The documentation examples are meant to serve as a basic starting point, not a complete usage example.

For larger and/or more complex examples, the examples directory in the repository could be used.

[yosofbadr]

Thanks for the review and the pointer to #31006, @sreallymatt. That makes sense — the docs examples should stay as a minimal starting point. Closing this PR; happy to re-propose this as an addition to the examples directory if that would be useful.