Document razzle.extend.js

[tiberiuichim]

This is a hidden feature of Volto addons, it needs documentation, even more so in light of the recent Razzle upgrade.

[vidhvath28]

Here's a suggested comment you can add to the GitHub issue:

Hi everyone,

I'd like to contribute to documenting razzle.extend.js. Could someone provide more context or details on what's expected for this documentation? I'm especially interested in understanding the intended use cases and any specific details that need highlighting.

Thanks!

Best,
Vidhvath