Add an example of continuously listening to a TCP stream in StreamPeerTCP

**Your Godot version:** 3.2.3

**Issue description:**

We should add an example of continuously listening to a TCP stream in the StreamPeerTCP class reference.

> For the stream itself, I am very indebted to [this little project](https://github.com/stisa/godot-tcp-example/tree/master/godot_tcp), as it showed me that you can use the process function in Godot to periodically listen to the TCP stream, which was a connection I hadn’t been able to make from the documentation. After I got that done, I still had some trouble getting the server to accept my data… turns out Godot’s convenience functions for strings and others helpfully prefixes it with a few bytes to indicate the size of the sent data. Using `put_data(string.to_utf8())` for this solved everything. Now the server was accepting my stream!
> […]
> The example of the TCP stream above would make a really good sample for the StreamPeer pages, like, giving users the idea they could use the process function to listen for incoming messages.

See https://wolthera.info/2020/12/xmpp-client-in-godot/.

**URL to the documentation page (if already existing):** https://docs.godotengine.org/en/latest/classes/class_streampeertcp.html