Add documentation to CustomItemPlugin

lib/src/main/kotlin/foundation/esoteric/minecraft/plugins/library/item/CustomItemPlugin.kt

@@ -2,6 +2,16 @@ package foundation.esoteric.minecraft.plugins.library.item
 
 import org.bukkit.plugin.Plugin
 
+/**
+ * Represents a plugin that implements custom items.
+ *
+ * You must implement this interface if you wish to use custom items in your plugin.
+ */
 interface CustomItemPlugin : Plugin {
+    /**
+     * The custom item manager associated with this plugin.
+     *
+     * A plugin must implement CustomItemPlugin to use the custom item manager.
+     */
     val customItemManager: CustomItemManager
 }