ICU-22921 Add howto guide to try MF 2.0 final candidate draft impls

See unicode-org#3334

docs/userguide/format_parse/messages/hello_mf2.cpp

@@ -0,0 +1,34 @@
+#include <iostream>
+
+#include "unicode/utypes.h"
+#include "unicode/calendar.h"
+#include "unicode/errorcode.h"
+#include "unicode/locid.h"
+#include "unicode/messageformat2.h"
+
+using namespace icu;
+
+int main() {
+    ErrorCode errorCode;
+    UParseError parseError;
+
+    icu::Calendar* cal(Calendar::createInstance(errorCode));
+    cal->set(2025, Calendar::JANUARY, 28);
+    UDate date = cal->getTime(errorCode);
+
+    message2::MessageFormatter::Builder builder(errorCode);
+    message2::MessageFormatter mf = builder
+            .setPattern("Hello {$user}, today is {$now :date style=long}!", parseError, errorCode)
+            .setLocale(Locale("en_US"))
+            .build(errorCode);
+
+    std::map<UnicodeString, message2::Formattable> argsBuilder;
+    argsBuilder["user"] = message2::Formattable("John");
+    argsBuilder["now"] = message2::Formattable::forDate(date);
+    message2::MessageArguments arguments(argsBuilder, errorCode);
+
+    icu::UnicodeString result = mf.formatToString(arguments, errorCode);
+    std::string strResult;
+    result.toUTF8String(strResult);
+    std::cout << strResult << std::endl;
+}