refactor(securityinsights): migrate to unified folder structure

[zedy-wj]

This PR migrates the securityinsights specification to the new unified folder structure according to Azure folder structure guidelines.

Changes:

• Consolidated TypeSpec and Swagger files under resource-manager/Microsoft.SecurityInsights/SecurityInsights
• Consolidated TypeSpec and Swagger files under data-plane/ThreatIntelligence
• Add data-plane/ThreatIntelligence/suppressions.yaml file.
• Add resource-manager/Microsoft.SecurityInsights/SecurityInsights/suppressions.yaml file.
• Fixed input-file paths in readme.md
• Update path in cspell.yaml file.
• Verified TypeSpec compilation works correctly

Benefits:

• Unified structure for better maintainability
• Aligns with Azure folder structure standards
• Maintains backward compatibility
• Supports future TypeSpec migration efforts

[github-actions]

Next Steps to Merge

Next steps that must be taken to merge this PR:

• ❌ Your PR requires an API stewardship board review as it introduces a new API version (label: new-api-version). Schedule the review by following aka.ms/azsdk/onboarding/restapischedule.

Comment generated by summarize-checks workflow run.

[github-actions]

API Change Check

APIView identified API level changes in this PR and created the following API reviews

Language	API Review for Package
Swagger	Microsoft.SecurityInsights-SecurityInsights

[zedy-wj]

Added label to suppress the Swagger SemanticValidation ci error. This error is not thrown by the latest version.

[qiaozha]

Hold on this PR for this time, there's some problem with the common folder need to be further verified.