Okay, this is my basic stab at how it works now.
In you custom components folder, you’ll need to create a subfolder called hello_world (or whatever you want to call it.
inside the hello_world folder, create based on what the component is… I am not sure what the hello-world is considered (sensor, notify, etc.) but if it is a sensor you would create sensor.py and put the code there.
You also need the init.py file in the hello_world folder… and possibly the manifest.json…
Take all this with a grain of salt… I am basing this off what I see in the custom components I am using and what I have seen in release notes.
Anyway, I expect the document is outdated and needs a bit of work to be up with all the changes that have been made that effect custom components.