chore: update .gitignore and build.gradle.kts for local server testing

- Added entries to .gitignore for local Paper test server and API documentation.
- Included the run-paper plugin in build.gradle.kts for local server execution.
- Configured runServer task with Minecraft version 1.21.4 for testing.

settings.gradle.kts

@@ -1 +1,8 @@
+pluginManagement {
+    repositories {
+        gradlePluginPortal()
+        mavenCentral()
+    }
+}
+
 rootProject.name = "LoChat"