This part of the documentation has still to be written. However, I have uploaded a first sample addon to this site. If you're logged in, go to the Document Exchange collection in downloads and check the new addons collection.
On another note: the reason I put the note in was to discourage people to fiddle with the extensions table as it is very powerful and can completely throw off the DMX installation. I will give a short example of extensibility and maybe that will help to understand.
In the DMX_Extensions table you will notice the fields ExtensionKey and ControlToLoad. Well, what happens when someone requests a particular entry (file, folder or hyperlink etc), is that the module first checks what kind of entry it is. Every entry has a reference to the ExtensionKey field in the DMX_Extensions table. Then, when the right extension has been identified, the ControlToLoad is inserted into the module. This means that, potentially, you can direct DMX to load a completely different control than normal. Normally the ViewDetails Control is loaded for regular files. This shows the version history, log, etc. However, you could make your own control and make it load that instead for, for instance, pdf files (incidentally the example that is currently available). This control could do something completely different. I've illustrated with the example that you could show the PDF in an iframe within the module, for instance. But you can easily see that you could extend it any way you like. You could potentially make a gallery addon, showing collections as thumbnail sheets, and image files directly.
This was just one way to extend it. There are others but work is still going on to test these features before documentation can be written. I hope you understand.