Update wording in documentation and clarify guideline warning

Rephrased a sentence in Introduction.md for better accuracy regarding NDCore usage.

Author: mtctx

.junie/guidelines.md

@@ -65,6 +65,8 @@ try (var executor = Executors.newVirtualThreadPerTaskExecutor()) {
 
 ### Structural Mandate
 
+DO NOT add any of those comments in the following code block to the real code:
+
 ```java
 // 1. Static variables
 private static final int MAX_PLAYERS = 100;

usage/Introduction.md

@@ -50,7 +50,7 @@ Additionally, there are standalone files for core components:
 - If you need to create rich text with colors and formatting, check the [TextBuilder documentation](builders/TextBuilder.md)
 - If you want to create custom menus, explore the [menu package documentation](menu/Introduction.md)
 - If you need to store persistent player data, see the [persistence package documentation](persistence/Introduction.md)
-- If you're extending NDCore with your own plugin, refer to the [NDPlugin documentation](NDPlugin.md)
+- If you're using NDCore in your own plugin, refer to the [NDPlugin documentation](NDPlugin.md)
 
 ## Contributing to Documentation