Refactor(policyinsights): migrate to unified folder structure

[v-xuto]

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

Change:

• Consolidated TypeSpec and Swagger files under resource-manager\Microsoft.PolicyInsights\PolicyInsights
• Fixed all common-types references and input-file paths in readme.md
• Added suppressions.yaml for legacy API versions
• 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]

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