Introduce checkstyle plugin

- Based on https://github.com/spring-io/spring-javaformat
- In this iteration, checkstyles are only enabled for spring-ai-core

document-readers/markdown-reader/pom.xml

@@ -1,4 +1,20 @@
 <?xml version="1.0" encoding="UTF-8"?>
+<!--
+  ~ Copyright 2023-2024 the original author or authors.
+  ~
+  ~ Licensed under the Apache License, Version 2.0 (the "License");
+  ~ you may not use this file except in compliance with the License.
+  ~ You may obtain a copy of the License at
+  ~
+  ~      https://www.apache.org/licenses/LICENSE-2.0
+  ~
+  ~ Unless required by applicable law or agreed to in writing, software
+  ~ distributed under the License is distributed on an "AS IS" BASIS,
+  ~ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  ~ See the License for the specific language governing permissions and
+  ~ limitations under the License.
+  -->
+
 <project xmlns="http://maven.apache.org/POM/4.0.0"
 		 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 		 xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">