Advertise our icon set to developers

Something we don't currently mention that might be important for developers coming from the web is our extensive system icon set. It might be worth looking at other sites that advertise icons to developers like FontAwesome, but here's some stuff I can think of that might be interesting:

* At this point, there's over 3,000 icon files (I'm not sure how many unique icon names).
* This covers both color icons and symbolic icons
* Maybe we can shoutout the fd.o spec and that we cover it?
* 16px, 24px, 32px, 48px, 64px, and 128px
* Symbolic icons are all SVG and be colored with Gtk.CSS
* Maybe link to this repo so you can explore?
* Actions, Devices, Mimetypes, Places, Status, etc

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No file, test, or entry point is named. Review the developer-facing content in appcenter-dashboard and compare it with the icon details and external examples listed in the issue; done means the icon set is clearly described with relevant links and supported categories, sizes, formats, and specifications.

Written by the indexing model from the issue text.