Docs: add DISCLAIMER file and pull request template

.github/pull_request_template.md

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+<!--Thank you for contributing! -->
+
+<!--In case of an existing issue or discussions, please reference it-->
+closes: #ISSUE_Number
+<!--Remove this section if no corresponding issue.-->
+
+---
+
+## Change logs
+
+> Describe your change clearly, including what problem is being solved or what document is being added or updated.
+
+## Contributor's checklist
+
+Here are some reminders before you submit your pull request:
+
+* Make sure that your Pull Request has a clear title and commit message. You can take the [Git commit template](https://github.com/apache/cloudberry/blob/main/.gitmessage) as a reference.
+* Learn the [code contribution](https://cloudberry.apache.org/contribute/code) and [doc contribution](https://cloudberry.apache.org/contribute/doc) guides for better collaboration.
+* Make sure that CICD workflow is successful.
+* List your communications in the [GitHub Issues](https://github.com/apache/cloudberry-backup/issues) or [Discussions](https://github.com/apache/cloudberry/discussions) (if has or needed).
+* Feel free to ask for the [cloudberry committers](https://github.com/orgs/apache/teams/cloudberry-committers) or other people to help review and approve.

DISCLAIMER

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+Apache Cloudberry is an effort undergoing incubation at The Apache
+Software Foundation (ASF), sponsored by the Apache
+Incubator. Incubation is required of all newly accepted projects until
+a further review indicates that the infrastructure, communications,
+and decision making process have stabilized in a manner consistent
+with other successful ASF projects. While incubation status is not
+necessarily a reflection of the completeness or stability of the code,
+it does indicate that the project has yet to be fully endorsed by the
+ASF.