Replace issue template with multiple issue forms.

.github/ISSUE_TEMPLATE/bug_report.yml

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+name: Bug report
+description: Report an unexpected behavior.
+labels: ["kind: bug", "needs: triage"]
+body:
+- type: markdown
+  attributes:
+    value: |
+      Thank you for your contribution.
+
+      It is helpful to explain how to reproduce the bug and what the problem is.
+      If you have a small reproducible example, you can use the second field to provide it.
+      Otherwise, please provide a link to a repository, a gist (https://gist.github.com) or drag-and-drop a `.zip` archive in the first field.
+- type: textarea
+  attributes:
+    label: Description of the problem
+    placeholder: What happens and what you would have expected instead.
+- type: textarea
+  attributes:
+    label: Small Coq file to reproduce the bug
+    placeholder: |
+      Goal True.
+        ok tactic.
+        buggy tactic.
+        (* the last line raises an error or an anomaly *)
+    render: coq
+- type: input
+  attributes:
+    label: Version of Coq where this bug occurs
+    description: |
+      You can get this information by running `coqtop -v`.
+      Feel free to provide a comma-separated list or a range of versions if you can reproduce the bug on several versions of Coq.
+    placeholder: 8.X.Y
+- type: input
+  attributes:
+    label: Last version of Coq where the bug did not occur
+    description: You can fill this optional field if the bug is a regression compared to a previous version of Coq.

.github/ISSUE_TEMPLATE/config.yml

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+blank_issues_enabled: true
+contact_links:
+  - name: Coq Zulip Chat
+    url: https://coq.zulipchat.com
+    about: For casual and high traffic discussions. You can ask questions about Coq (#Coq users) or questions about Coq internals (#Coq devs & plugin devs). There are also many streams dedicated to external Coq projects.
+  - name: Coq Discourse Forum
+    url: https://coq.discourse.group
+    about: Our official, multilingual forum, for more structured and easy to search discussions. You can also ask about Coq or internals here, and this is the place to send any announcements.
+  - name: Proof Assistants Stack Exchange
+    url: https://proofassistants.stackexchange.com
+    about: For questions about Coq, Lean, Agda, Isabelle, etc. This is a Q&A site, where numerous Coq users are available to answer questions, but this is not an official Coq forum.

.github/ISSUE_TEMPLATE/feature_request.yml

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+name: Feature request
+description: Suggest an idea of improvement.
+labels: ["kind: wish", "needs: triage"]
+body:
+- type: textarea
+  attributes:
+    label: Is your feature request related to a problem?
+    placeholder: A clear and concise description of what the problem is. E.g., I'm frustrated when [...]
+
+- type: textarea
+  attributes:
+    label: Proposed solution
+    placeholder: Describe your ideal solution.
+
+- type: textarea
+  attributes:
+    label: Alternative solutions
+    placeholder: List any alternative solutions or features you've considered.
+
+- type: textarea
+  attributes:
+    label: Additional context
+    placeholder: Add any other context or screenshots about the feature request here.