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.

build.gradle.kts

@@ -1,5 +1,6 @@
 plugins {
     java
+    id("xyz.jpenilla.run-paper") version "2.3.1"
 }
 
 group = "com.loki"
@@ -52,3 +53,9 @@ tasks.jar {
     from(configurations.runtimeClasspath.get().map { if (it.isDirectory) it else zipTree(it) })
     duplicatesStrategy = DuplicatesStrategy.EXCLUDE
 }
+
+// Локальный Paper для тестов: .\gradlew.bat runServer
+tasks.runServer {
+    minecraftVersion("1.21.4")
+    // Совпадает с compileOnly("io.papermc.paper:paper-api:1.21.4-R0.1-SNAPSHOT")
+}