Migrate issue templates to forms (#870)

.github/ISSUE_TEMPLATE/bug.yml

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+# This form has been adapted from https://github.com/conda/conda/blob/main/.github/ISSUE_TEMPLATE/0_bug.yml.
+name: Bug report
+description: "`message_ix` does not work as expected."
+body:
+  - type: markdown
+    attributes:
+      value: |
+        Because processing new bug reports is time-consuming, we would like to ask you to fill out the following form to the best of your ability and as completely as possible.
+
+        > [!NOTE]
+        > If you have a question about how to *use* message_ix or ixmp, please first:      
+        > - Read the [Prerequisite knowledge & skills](https://docs.messageix.org/en/stable/prereqs.html) page of the documentation.
+        > - Check the [Discussions](https://github.com/iiasa/message_ix/discussions) associated with the message_ix GitHub repository.
+  - type: textarea
+    id: what
+    attributes:
+      label: What happened?
+      description: Please tell us what happened. Please provide as many details as possible. The more information provided, the more likely we are able to replicate your problem and offer a solution.
+    validations:
+      required: true
+  - type: textarea
+    id: info
+    attributes:
+      label: Code Sample
+      description: If possible, provide a minimal, working code example below. Your example should work in itself up until the point where it produces the error you are reporting.
+      render: python
+  - type: textarea
+    id: expectation
+    attributes:
+      label: What did you expect to happen?
+      description: Please tell us what you expected to happen.
+    validations:
+      required: true
+  - type: textarea
+    id: versions
+    attributes:
+      label: Versions
+      description: |
+        The exact package versions you installed into your environment can hold the solution to the problem you are facing.
+
+        Please activate the environment within which you are encountering this bug, run the following command from your command line, and paste the output below.
+
+        ```bash
+        message-ix show-versions
+        ```
+
+        Alternatively, activate your environment and run the following in a Python interpreter:
+        ```python
+        import ixmp; ixmp.show_versions()
+        ```
+      render: shell
+  - type: textarea
+    id: context
+    attributes:
+      label: Additional Context
+      description: Please include any additional information (or screenshots) that you think would be valuable.

.github/ISSUE_TEMPLATE/config.yml

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+blank_issues_enabled: false

.github/ISSUE_TEMPLATE/other.yml

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+name: Other issue
+description: "Feature request, enhancement proposal, etc."
+body:
+  - type: markdown
+    attributes:
+      value: |
+        > [!NOTE]
+        > If you have a question about how to *use* message_ix or ixmp, please first:      
+        > - Read the [Prerequisite knowledge & skills](https://docs.messageix.org/en/stable/prereqs.html) page of the documentation.
+        > - Check the [Discussions](https://github.com/iiasa/message_ix/discussions) associated with the message_ix GitHub repository.
+
+        If these do not help, please describe your issue below.
+  - type: textarea
+    id: what
+    attributes:
+      label: What is this about?
+      description: Please tell us what this issue is about. Please provide as many details as possible. The more information you provide, the more likely we are able to help you or incorporate your suggestions!
+    validations:
+      required: true