docs(examples): add graph-traversal-viewer MCP chat app use-case

A validating MDL build script for the OntologyViewer module — a conversational
AI assistant that queries a knowledge graph via an MCP client and embeds a
custom GraphTraversalViewer widget. Demonstrates the full pipeline (enums,
entities + access, associations, java actions, microflows, nanoflow, snippet,
chat page with the custom pluggable widget) in one script.

Header documents the marketplace prerequisites (the agent/conversational-AI
stack + the custom widget) and how to install them via marketplace install.
Moved from examples/mdl/ into mdl-examples/use-cases/ alongside the other
end-to-end demos. Checks clean (49 statements).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: ako