Add mention of gtk4-icon-browser for figuring out available icons to docs
First up, thanks for making the library! I'm new to GUI toolkits like GTK/QT (originally webdev) and I've been playing around with this for a couple hours and had quite a bit of fun with how quickly and easily everything fit together, particularly how easy it was to split Widgets into their own files and have them share data!

I was fiddling around with the todo-list tutorial and I noticed it using a couple icons here or there. That got me wondering what the available icons were. After 15 minutes of googling and some luck I stumbled over a mention of gtk4-icon-browser. A bit of more googling later on how to install that and I had a chance to see what icons there are. It would be nice if we could shorten that link from "Oh, what icons can I have?" to finding that list for newcomers such as myself.

Imo it makes sense to  mention in the docs what tooling there is out there that one likely needs, such as gtk4-icon-browser to see a list of available icons. If there are more things out there that one should know about I'd love to of course also know about those ^^ 

I'd be happy to write a 3-4 line paragraph for this, but I'm not sure where this should be mentioned. A separate "recommended-tools.md" ? As part of "widgets.md" at the bottom? 