Reupload preface extension section

mikebo93 · mikebo93 · commit c6243cbd268c · 2025-03-26T22:36:54.000Z
diff --git a/chapter_preface_extension/Index.md b/chapter_preface_extension/Index.md
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+# Part II Application Scenarios {#part-ii-application-scenarios .unnumbered}
+
+In Part II, we will introduce various scenarios of applying machine
+learning frameworks. These scenarios include federated learning systems,
+recommender systems, reinforcement learning systems, and robotic
+systems.
