[FLINK-40772][docs] Add a PR template with AI disclosure

.github/PULL_REQUEST_TEMPLATE.md

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+<!--
+Licensed to the Apache Software Foundation (ASF) under one
+or more contributor license agreements.  See the NOTICE file
+distributed with this work for additional information
+regarding copyright ownership.  The ASF licenses this file
+to you 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
+
+  http://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.
+-->
+
+<!--
+Thank you for contributing to Apache Flink Training.
+Please follow the repository contribution guide:
+https://github.com/apache/flink-training/blob/master/CONTRIBUTING.md
+and the Flink contribution guide:
+https://flink.apache.org/how-to-contribute/contribute-code/
+
+Link the corresponding Jira issue below.
+
+Trivial hotfixes, such as typos or syntax errors, need no Jira issue.
+Use a title in the form "[hotfix] Description".
+-->
+
+## Purpose of the change
+
+<!-- Link the Jira issue (if any) and explain the problem this change addresses. -->
+
+## Brief change log
+
+<!-- Summarize changes to exercises, solutions, tests, or training documentation. -->
+
+## Verifying this change
+
+<!-- Describe the tests or manual checks performed and their results. -->
+
+## Impact and documentation
+
+<!--
+Describe any changes to exercise behavior, prerequisites, or learner setup.
+Explain how affected instructions and reference solutions have been updated.
+If neither applies, state "Not applicable".
+-->
+
+---
+
+##### Was generative AI tooling used to co-author this PR?
+
+<!--
+If generative AI tooling has been used in the process of authoring this PR, please
+change the checkbox below to `[X]` and replace the placeholder in the "Generated-by"
+line with the tool name and version. Otherwise remove the "Generated-by" line.
+See the ASF Generative Tooling Guidance for details:
+https://www.apache.org/legal/generative-tooling.html
+
+You are responsible for the quality and correctness of every change in this PR
+regardless of the tooling used. Low-effort AI-generated PRs will be closed. See
+https://flink.apache.org/how-to-contribute/contribute-code/#ai-assisted-contributions
+for the full guidance.
+-->
+
+- [ ] Yes (please specify the tool below)
+
+Generated-by: [Tool Name and Version]