Update issue templates

.github/ISSUE_TEMPLATE/--work-item--dev-only-.md

+---
+name: "\U0001F528Work Item (DEV ONLY)"
+about: Work item issue for tracking progress. Dev team only.
+title: ''
+labels: Work Item
+assignees: ''
+
+---
+
+## 🔨Work Item
+
+**IMPORTANT:**
+* This template is only for dev team to track project progress. For feature request or bug report, please use the corresponding issue templates.
+* DO NOT create a new work item if the purpose is to fix an existing issue or feature request. We will directly use the issue in the project tracker.
+
+Project tracker: https://github.com/orgs/dmlc/projects/2
+
+## Description
+
+<!-- short description of the work item -->
+
+## Depending work items or issues
+
+<!-- what must be done before this -->

.github/ISSUE_TEMPLATE/bug-report.md

 ---
 name: "\U0001F41B Bug Report"
 about: Submit a bug report to help us improve DGL
+title: ''
+labels: ''
+assignees: ''
 
 ---
 

.github/ISSUE_TEMPLATE/documentation.md

 ---
 name: "\U0001F4DA Documentation"
 about: Report an issue related to docs.dgl.ai
+title: ''
+labels: ''
+assignees: ''
 
 ---
 

.github/ISSUE_TEMPLATE/feature-request.md

 ---
 name: "\U0001F680Feature Request"
 about: Submit a proposal/request for a new DGL feature
+title: ''
+labels: ''
+assignees: ''
 
 ---
 

.github/ISSUE_TEMPLATE/questions-help-support.md

 ---
 name: "❓Questions/Help/Support"
 about: Do you need support? We have resources.
+title: ''
+labels: ''
+assignees: ''
 
 ---
 

.github/ISSUE_TEMPLATE/work-item.md

+---
+name: Work Item
+about: Describe this issue template's purpose here.
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+