Improve Custom Font Documentation

[HoofedEar]

https://docs.avaloniaui.net/docs/guides/styles-and-resources/how-to-use-fonts

This guide could use more details about how the notation here works, for App.axaml:

<Application.Resources>
        <FontFamily x:Key="NunitoFont">avares://GoogleFonts/Assets/Fonts#Nunito</FontFamily>
</Application.Resources>

There are no details about how the #Nunito portion works. Since there is not file named Nunito.ttf in the GoogleFonts/Assets/Fonts folder, it could be assumed that Nunito is the name of the font. However, how would this work with a font that has spaces in its name? Etc etc. It would be useful to have a more in-depth explanation for requirements on the syntax.