docs/design: update template

[zz-jason]

What problem does this PR solve?

Problem Summary: the current design doc template is not detailed enough and lacks directivity. Update the template to make it clearer.

What is changed and how it works?

What's Changed: design doc template is updated.

Related changes

• PR to update pingcap/docs/pingcap/docs-cn: N/A

Check List

Tests

• No code

Side effects

Release note

• N/A

[zz-jason]

@bb7133 @XuHuaiyu PTAL

[zz-jason]

/cc @XuHuaiyu @bb7133

[bb7133]

/lgtm

[ti-chi-bot]

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[XuHuaiyu]

LGTM

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• XuHuaiyu
• bb7133

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[zz-jason]

/merge

[ti-chi-bot]

@zz-jason: It seems you want to merge this PR, I will help you trigger all the tests:

/run-all-tests

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[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: b2267a9

[ti-chi-bot]

@zz-jason: Your PR was out of date, I have automatically updated it for you.

At the same time I will also trigger all tests for you:

/run-all-tests

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the ti-community-infra/tichi repository.