Fix tutorial example (trac #11017)

Extract example into a C++ file, so that it can be tested, unfortunately this
means that it no longer links to the reference documentation.

doc/samples/tutorial.cpp

@@ -0,0 +1,27 @@
+#include <boost/container_hash/hash.hpp>
+#include <vector>
+#include <algorithm>
+#include <iterator>
+#include <cassert>
+
+//[ get_hashes
+template <class Container>
+std::vector<std::size_t> get_hashes(Container const& x)
+{
+    std::vector<std::size_t> hashes;
+    std::transform(x.begin(), x.end(), std::back_inserter(hashes),
+        boost::hash<typename Container::value_type>());
+
+    return hashes;
+}
+//]
+
+int main() {
+    std::vector<int> values;
+    values.push_back(10);
+    values.push_back(20);
+
+    std::vector<std::size_t> hashes = get_hashes(values);
+    assert(hashes[0] = boost::hash<int>()(values[0]));
+    assert(hashes[1] = boost::hash<int>()(values[1]));
+}