[examples] Add compilable code snippets (#7909)

This enables frc-docs to use RLIs for things that are currently in-line
code blocks, and ensures they compile, which is important with the 2027
breaking changes coming. They are kept separate from the examples to
ensure they don't polute the VSCode examples finder.

Adds the Encoder snippets used in the frc-docs Encoder article as the
first instance of this.

wpilibcExamples/src/main/cpp/snippets/snippets.json

@@ -0,0 +1,12 @@
+[
+  {
+    "name": "Encoder",
+    "description": "Snippets of Encoder class usage for frc-docs.",
+    "tags": [
+      "Hardware",
+      "Encoder"
+    ],
+    "foldername": "Encoder",
+    "gradlebase": "cpp"
+  }
+]