feat(examples): add a hello project with a library and a consumer

Smallest project that exercises the whole pipeline: one static library and one
executable that links against it.

examples/hello/src/main.c

@@ -0,0 +1,18 @@
+#include "mathx.h"
+#include <stdio.h>
+
+#ifndef GREETING
+#define GREETING "hello"
+#endif
+
+int main(void) {
+    double xs[] = {1.0, 2.0, 3.0, 4.0, 5.0};
+    size_t n = sizeof xs / sizeof xs[0];
+
+    printf("%s\n", GREETING);
+    printf("  dot    %.3f\n", vec_dot(xs, xs, n));
+    printf("  norm   %.3f\n", vec_norm(xs, n));
+    printf("  mean   %.3f\n", stats_mean(xs, n));
+    printf("  stddev %.3f\n", stats_stddev(xs, n));
+    return 0;
+}