Remove remaining references to the three cloud storage extensions from documentation data and Doxygen files. This ensures the generated documentation no longer mentions the removed extensions.

Tasks
1. Remove arch-cloud-storage-extension entry from dist/docs_data.py
2. Remove @section s3_store S3 Storage source section and @section azure_gcp_store Azure and GCP Storage Source section from src/docs/arch-storage-extension.dox
3. Replace s3_store example in src/docs/arch-tiered-storage.dox with dir_store example

Definition of Done
- arch-cloud-storage-extension entry is removed from dist/docs_data.py
- S3 and Azure/GCP sections are removed from src/docs/arch-storage-extension.dox
- S3 example is replaced with dir_store in src/docs/arch-tiered-storage.dox
- dist/s_all validation passes (regenerates documentation without errors)
- Documentation builds successfully without references to removed extensions