docs: update workshop README for notebook-first content model

Rewrite the CX Discovery Workshop README to reflect the architecture
shift where content lives in the notebook's `topic-bank` cell rather
than in `discoverylib/topics.py`. The library now holds code only, the
notebook is the deliverable (no longer generated via build_notebook.py),
and tests pin content read directly from the notebook. Update layout,
run, and extending sections accordingly.

studies/202607_CX_Discovery_Workshop/pyproject.toml

@@ -17,6 +17,7 @@ dependencies = [
     "jupyterlab>=4.0",
     "ipywidgets>=8.0",
     "nbconvert>=7",
+    "nbformat>=5.9",
     "tabulate>=0.9",
 ]