Split Demo 1 guides by platform

[justinyoo]

Summary

Split the combined Demo 1 documentation into independent Java and .NET runbooks and validation guides. Update setup guidance and repository indexes so each platform has a coherent end-to-end path with platform-specific commands and troubleshooting.

Change type

• Presentation content
• Presentation design
• PowerShell
• Bash
• Bicep
• Documentation or repository maintenance

Validation

• Marp HTML generation succeeds.
• Updated slides were visually reviewed.
• PowerShell files parse without errors.
• Bash files pass bash -n.
• Bicep files build successfully.
• No generated files, credentials, or sensitive data are included.
• Markdown diagnostics report no errors.
• git diff --check passes.
• Removed guide paths have no remaining Markdown references.

Screenshots

Not applicable; documentation structure and commands only.

Related issues

None.