refactor(mongocluster): migrate to unified folder structure

[v-xuto]

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

Changes Made

• Consolidated TypeSpec and Swagger files under resource-manager/Microsoft.DocumentDB/MongoCluster/
• Updated tspconfig.yaml with correct relative paths.
• Created suppressions.yaml for TypeSpec requirements.
• Verified TypeSpec compilation works correctly.

Benefits

• Complies with Azure folder structure guidelines.
• Maintains all existing functionality.
• Preserves all API versions.
• No breaking changes to consumers.

[github-actions]

Next Steps to Merge

Next steps that must be taken to merge this PR:

• ❌ This PR targets either the main branch of the public specs repo or the RPSaaSMaster branch of the private specs repo. These branches are not intended for iterative development. Therefore, you must acknowledge you understand that after this PR is merged, the APIs are considered shipped to Azure customers. Any further attempts at in-place modifications to the APIs will be subject to Azure's versioning and breaking change policies. Additionally, for control plane APIs, you must acknowledge that you are following all the best practices documented by ARM at aka.ms/armapibestpractices. If you do intend to release the APIs to your customers by merging this PR, add the PublishToCustomers label to your PR in acknowledgement of the above. Otherwise, retarget this PR onto a feature branch, i.e. with prefix release- (see aka.ms/azsdk/api-versions#release--branches).
• ❌ This PR has at least one breaking change (label: BreakingChangeReviewRequired).
  To unblock this PR, follow the process at aka.ms/brch.
• ❌ The required check named TypeSpec Validation has failed. Refer to the check in the PR's 'Checks' tab for details on how to fix it and consult the aka.ms/ci-fix guide

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
TypeSpec	Microsoft.DocumentDB
C#	Azure.ResourceManager.MongoCluster
JavaScript	@azure/arm-mongocluster