faq: add icon fallback

src/guide/faq.mdx

@@ -52,6 +52,24 @@ inside of them.
 }
 ```
 
+### Get rid of the purple/black icons
+The @@Quickshell.Quickshell.iconPath() function has three variants:
+- One draws a purple/black square if the icon is missing.
+- One allows you to specify a fallback icon if the desired one is missing.
+- One returns an empty string if the icon is missing.
+
+Either of the last two variants can be used to avoid the purple/black square.
+
+Note that you can change out a loader's component conditionally:
+```qml
+@@QtQuick.Loader {
+  readonly property Component thing1: ...
+  readonly property Component thing2: ...
+
+  sourceComponent: condition ? thing1 : thing2
+}
+```
+
 ## Something is broken
 
 ### There is a hole in my window