docs: update pull request template for clarity and completeness

[Ki-Seki]

Description

Summary: Enhance the pull request template to provide clearer instructions and ensure all necessary information is included for reviewers.

Fix: #44

Reviewer: @J1awei-Yang

Checklist:

• I have performed a self-review of my own code | 我已自行检查了自己的代码
• I have commented my code in hard-to-understand areas | 我已在难以理解的地方对代码进行了注释
• I have added tests that prove my fix is effective or that my feature works | 我已添加测试以证明我的修复有效或功能正常
• I have added necessary documentation (if applicable) | 我已添加必要的文档（如果适用）
• I have linked the issue to this PR (if applicable) | 我已将 issue 链接到此 PR（如果适用）
• I have mentioned the person who will review this PR | 我已提及将审查此 PR 的人

[Copilot]

Pull Request Overview

Enhance the repository’s pull request template to give contributors clearer guidance and ensure all key fields are filled out.

• Introduce an HTML comment block with step-by-step instructions for summary, issue, and reviewer fields
• Add explicit Summary, Fixes, and Reviewer placeholders in the template
• Refine checklist entries and their Chinese translations for consistency

Comments suppressed due to low confidence (2)

.github/PULL_REQUEST_TEMPLATE.md:15

• [nitpick] The placeholder syntax (summary) may be overlooked; using a clearer marker like <summary> or YOUR_SUMMARY_HERE can improve visibility.

Summary: (summary)