Add template for documentation issues

Author: glatterf42

.github/ISSUE_TEMPLATE/config.yml

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+blank_issues_enabled: true

.github/ISSUE_TEMPLATE/project_paper.yml

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+name: Project or paper documentation
+description: Provide information about a project or paper for the MESSAGE team.
+title: "Document "
+labels: ["doc", "paper"]
+projects: ["MESSAGEix team"]
+body:
+  - type: markdown
+    attributes:
+      value: |
+        ## Thanks for taking the time to fill out this documentation issue!
+
+        Please complete the issue title with the name of the project or paper you're documenting.
+        Please answer as many of the following points as you can, but no worries if you don't have all the answers right away.
+        These prompts mainly serve as a reminder as to what people may want to know.
+        Feel free to update this issue whenever you have more answers and the time :smiley:
+        Please remember to close this issue once the project is complete.
+  - type: textarea
+    id: who
+    attributes:
+      label: Who is responsible?
+      description: Please tell us who's responsible for the project. If they have GitHub accounts, please assign them on this issue.
+      placeholder: Me, myself, and I.
+    validations:
+      required: true
+  - type: textarea
+    id: what
+    attributes:
+      label: What's the goal?
+      description: Please tell us what this project is about.
+    validations:
+      required: true
+  - type: textarea
+    id: when
+    attributes:
+      label: When does the project end?
+      description: Please tell us the timeframe of this project.
+  - type: textarea
+    id: where
+    attributes:
+      label: Where does the project live?
+      description: Please tell us where we can find the code and data this project is using.
+  - type: textarea
+    id: why
+    attributes:
+      label: Why is this project important?
+      description: If you feel so inclined, please provide background information.
+  - type: textarea
+    id: state
+    attributes:
+      label: What's the current state of the work?
+      description: "Please tell us what has been done already. If the project Scenario's follow a specific naming convention, this is a good place to highlight it :smiley:"
+  - type: textarea
+    id: more
+    attributes:
+      label: "For further information, see:"
+      description: Please tell us where we can learn more about this project.