docs: Update PR template to emphasize guidelines and issue linking (#20382)

Signed-off-by: -LAN- <laipz8200@outlook.com>

.github/pull_request_template.md

-# Summary
+> [!IMPORTANT]
+>
+> 1. Make sure you have read our [contribution guidelines](https://github.com/langgenius/dify/blob/main/CONTRIBUTING.md)
+> 2. Ensure there is an associated issue and you have been assigned to it
+> 3. Use the correct syntax to link this PR: `Fixes #<issue number>`.
 
-Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context. List any dependencies that are required for this change.
+## Summary
 
-> [!Tip]
-> Close issue syntax: `Fixes #<issue number>` or `Resolves #<issue number>`, see [documentation](https://docs.github.com/en/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-issue#linking-a-pull-request-to-an-issue-using-a-keyword) for more details.
+<!-- Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context. List any dependencies that are required for this change. -->
 
-
-# Screenshots
+## Screenshots
 
 | Before | After |
 |--------|-------|
 | ...    | ...   |
 
-# Checklist
-
-> [!IMPORTANT]  
-> Please review the checklist below before submitting your pull request.
+## Checklist
 
 - [ ] This change requires a documentation update, included: [Dify Document](https://github.com/langgenius/dify-docs)
 - [x] I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
 - [x] I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
 - [x] I've updated the documentation accordingly.
 - [x] I ran `dev/reformat`(backend) and `cd web && npx lint-staged`(frontend) to appease the lint gods
-