Doc: update security policy and PR template

[tuhaihe]

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Change logs

We update the security policy to make our handling process more clear. In addition, adding the code contribution guide to the PR template for developers reference.

Why are the changes needed?

Describe why the changes are necessary.

Does this PR introduce any user-facing change?

If yes, please clarify the previous behavior and the change this PR proposes.

How was this patch tested?

Please detail how the changes were tested, including manual tests and any relevant unit or integration tests.

Contributor's Checklist

Here are some reminders and checklists before/when submitting your pull request, please check them:

• Make sure your Pull Request has a clear title and commit message. You can take git-commit template as a reference.
• Sign the Contributor License Agreement as prompted for your first-time contribution.
• List your communication in the GitHub Issues or Discussions (if has or needed).
• Document changes.
• Add tests for the change
• Pass make installcheck
• Pass make -C src/test installcheck-cbdb-parallel
• Feel free to @cloudberrydb/dev team for review and approval when your PR is ready🥳