docs: add PR template

.github/PULL_REQUEST_TEMPLATE.md

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+<!-- Thank you for contributing! -->
+
+### Before submitting the PR, please make sure you do the following <!-- (put an "X" next to an item) -->
+
+- [ ] Read the [Contributing Guidelines](https://github.com/vuepress/vuepress-next/blob/main/docs/contributing.md).
+- [ ] Provide a description in this PR that addresses **what** the PR is solving. If this PR is going to solve an existing issue, please reference the issue (e.g. `close #123`).
+
+### What is the purpose of this pull request? <!-- (put an "X" next to an item) -->
+
+- [ ] Bug fix
+- [ ] New feature
+- [ ] Documentation update
+- [ ] Other
+
+### Description
+
+<!-- Please insert your description here and provide especially info about the "what" this PR is solving -->
+
+### Screenshots
+
+<!-- If your PR includes UI changes, please provide before/after screenshots. If there are any other images that add context to the PR, add them here as well -->
+
+**Before**
+
+**After**