docs(examples): production-facing context_graph README + refreshed codelab flow diagram (#302)

* docs(examples): restructure context_graph README for a production audience

Lead with the two SQL artifacts (table_ddl.sql + property_graph.sql) and the
one-flag 'bqaa context-graph --property-graph' flow, move the explicit
ontology/binding pipeline below an Advanced divider, and remove internal
references (PR/issue links, gist link, test project IDs, per-beat run
evidence).

* docs(codelabs): refresh context-graph-flow diagram

* docs(blog): refresh context-graph-flow diagram

* docs(codelab): regenerate colab notebook with refreshed flow diagram

Author: caohy1988