[ci skip] Add examples README

examples/README.md

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+This directory contains examples that use different parts of ArborX.
+
+Directory | Description
+|:-- |:-- |
+`access_traits`         | provide non-View user data to ArborX
+`brute_force`           | use brute force to solve a problem
+`callback`              | execute search which calls user function on match
+`dbscan`                | cluster a set of points using DBSCAN
+`molecular_dynamics`    | do one step of moving particles under forces
+`moving_least_squares`  | interpolate data between point clouds using MLS
+`raytracing`            | deposit ray energy onto intersected boxes
+`simple_intersection`   | find boxes that contain points
+`triangle_intersection` | find triangles that contain points
+`viz`                   | visualize hierarchy and traversal