Rewrite all concepts documentation

hello RC documentation

docs/guides/concepts/samples/entities.cs

@@ -0,0 +1,13 @@
+public string GetChannelTopic(ulong id)
+{
+	var channel = client.GetChannel(81384956881809408) as SocketTextChannel;
+	if (channel == null) return "";
+	return channel.Topic;
+}
+
+public string GuildOwner(SocketChannel channel)
+{
+	var guild = (channel as SocketGuildChannel)?.Guild;
+	if (guild == null) return "";
+	return Context.Guild.Owner.Username;
+}