chore: update PR template

[BugenZhao]

I hereby agree to the terms of the RisingWave Labs, Inc. Contributor License Agreement.

What's changed and what's your intention?

• Vendor CLA into our repository.
• Mark several task list item as OPTIONAL so that task-list-completed job does not require them for a green checkmark.
• Comment out instructive texts in the task list to make it more concise.
• Remove redundant information in the documentation section.
• Fix orphan close tag </details> for the release note section and make it foldable.

Checklist

• I have written necessary rustdoc comments.
• I have added necessary unit tests and integration tests.
• I have added test labels as necessary.
• I have added fuzzing tests or opened an issue to track them.
• My PR contains breaking changes.
• My PR changes performance-critical code, so I will run (micro) benchmarks and present the results.
• My PR contains critical fixes that are necessary to be merged into the latest release.

Documentation

• My PR needs documentation updates.

Release note

[xxchan]

This may break the action

risingwave/.github/workflows/auto-create-doc-issue-by-pr.yml

Line 28 in ffe63f1

if [[ $PR_BODY == *"- [x] My PR needs documentation updates."* ]]; then