[Docs] Update issue template of documentation (#822)

* update docs template

* Update .github/ISSUE_TEMPLATE/3-documentation.yml

Co-authored-by: Zaida Zhou <58739961+zhouzaida@users.noreply.github.com>

.github/ISSUE_TEMPLATE/3-documentation.yml

+name: 📚 Documentation
+description: Report an issue related to the documentation.
+labels: "docs"
+title: "[Docs] "
+
+body:
+  - type: markdown
+    attributes:
+      value: |
+        ## Note
+        For general usage questions or idea discussions, please post it to our [**Forum**](https://github.com/open-mmlab/mmengine/discussions)
+        Please fill in as **much** of the following form as you're able to. **The clearer the description, the shorter it will take to solve it.**
+
+  - type: textarea
+    attributes:
+      label: 📚 The doc issue
+      description: >
+        A clear and concise description the issue.
+    validations:
+      required: true
+
+  - type: textarea
+    attributes:
+      label: Suggest a potential alternative/fix
+      description: >
+        Tell us how we could improve the documentation in this regard.
+
+  - type: markdown
+    attributes:
+      value: |
+        ## Acknowledgement
+        Thanks for taking the time to fill out this report.
+
+        If you have already identified the reason, we strongly appreciate you creating a new PR to fix it [**here**](https://github.com/open-mmlab/mmengine/pulls)!
+        Please refer to [Contribution Guide](https://mmengine.readthedocs.io/en/latest/notes/contributing.html) for contributing.
+
+        Welcome to join our [**Community(TODO)**](https://mmengine.readthedocs.io/en/latest/contact.html) to discuss together. 👬