docs: reorganize the Use Cases tab by what each example builds

The tab is Use Cases now, grouped General, Search, RAG, Feature Engineering,
Training, World Models -- one page per example, named for the example rather
than for its position in a tree.

The gallery pages that listed external notebooks become the More Examples entry
of the category they belong to, instead of standing in front of the real
tutorials. The top-level overview stops linking to itself through three levels
of index page and names the categories directly.

Branch experiments move to Feature Engineering, which is what they are an
experiment in. The agent plugin leads General, since it applies to everything
else in the tab.

LeRobotDataset moves to the Datasets tab, where the LeRobot datasets already
are, and Robotics moves to the top of that tab.

docs/web/tutorials/feature-engineering/index.mdx

@@ -1,7 +1,7 @@
 ---
-title: "Feature Engineering"
-sidebarTitle: "Feature Engineering"
-description: "Learn how to build features for your data in LanceDB."
+title: "More feature engineering examples"
+sidebarTitle: "More Examples"
+description: "Notebooks covering derived features and materialized views."
 layout: wide
 ---