add intents docs and samples

samples/msg-sender.lua

@@ -0,0 +1,17 @@
+function on_resume()
+    ui:show_lines{
+        "send text",
+        "send 123",
+        "send table",
+    }
+end
+
+function on_click(idx)
+    if idx == 1 then
+        system:send_message("text", "msg-receiver.lua")
+    elseif idx == 2 then
+        system:send_message(1)
+    else
+        system:send_message{ "one", "two", "three" }
+    end
+end