Conduct a comprehensive review of all secondary storage/RDBMS overview content across the documentation and add visual diagrams where appropriate to clarify architecture, data flow, and design decisions. This includes database concepts pages, deployment overview pages, and related architectural documentation.

Goals

• Add visual diagrams to key secondary storage overview pages to supplement text explanations
• Clarify how RDBMS, Elasticsearch, and OpenSearch integrate with the Orchestration Cluster
• Establish consistent visual representation of secondary storage architecture across all documents
• Reduce reliance on lengthy text explanations by using visual references

Scope

Review and update the following overview pages with diagrams as appropriate:

• /docs/self-managed/concepts/databases/overview.md - Secondary storage concept overview
• /docs/self-managed/concepts/databases/relational-db/index.md - RDBMS introduction (new page)
• /docs/self-managed/concepts/secondary-storage/index.md - Secondary storage architecture page
• /docs/self-managed/deployment/manual/rdbms/index.md - Manual RDBMS deployment overview (existing diagram may need review)
• /docs/self-managed/deployment/helm/install/helm-with-rdbms.md - Helm RDBMS overview section

Suggested diagram types

1. Architecture diagrams - Zeebe broker + secondary storage relationships
2. Data flow diagrams - Write paths (exporters) vs read paths (OC components)
3. Topology diagrams - Component relationships