docs: document contents of binary files

test/testWorkspace/lib/circleArea.fpkg.txt

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+circleArea.fpkg contains a single function that computes the area of a circle given its radius:
+
+```
+/// Compute the area of a circle given its radius.
+def circleArea(r: Int32): Int32 = 3 * (r * r)
+```

test/testWorkspace/lib/external/SquareArea.jar.txt

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+SquareArea.jar contains the compiled class file for the following Java code:
+
+```
+package dev.flix;
+
+public class SquareArea {
+  /**
+   * Compute the area of a square given its side length.
+   */
+  public static int squareArea(int w) { return w * w; }
+}
+```