[Docs] Including custom symbolic icons is different in GTK 4

[Mabi19]

Describe the bug
In GTK 4, icons need to be put in a faux-icon-theme directory structure to be recognized as symbolic - just putting them in there like in GTK 3 will not recolor them anymore.

To Reproduce
N/A

Expected behavior
The docs should say that in GTK 4 you have to follow the icon theme directory structure.

Additional context
Here's an example directory structure that works:

icons
└── hicolor
    └── scalable
        ├── devices
        │   ├── fa-memory-symbolic.svg
        │   └── fa-microchip-symbolic.svg
        └── status
            ├── fa-bell-slash-symbolic.svg
            └── fa-bell-symbolic.svg

Previously, all 4 of the SVG files would just be put in the icons directory directly.