Set up GitHub Discussions with category templates

.github/DISCUSSION_TEMPLATE/general.yml

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+title: "[General] "
+labels: []
+body:
+  - type: markdown
+    attributes:
+      value: |
+        Use this space for general discussions about simulation theory, the repository, or anything else you'd like to share with the community.
+  - type: textarea
+    id: discussion
+    attributes:
+      label: What would you like to discuss?
+      description: Share your thoughts, questions, or ideas with the community.
+      placeholder: Start your discussion here...
+    validations:
+      required: true

.github/DISCUSSION_TEMPLATE/ideas.yml

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+title: "[Idea] "
+labels: []
+body:
+  - type: markdown
+    attributes:
+      value: |
+        Share an idea for improving the repository, adding new content, or exploring new aspects of simulation theory.
+  - type: textarea
+    id: idea
+    attributes:
+      label: Idea Description
+      description: Describe your idea in detail.
+      placeholder: My idea is...
+    validations:
+      required: true
+  - type: textarea
+    id: motivation
+    attributes:
+      label: Motivation
+      description: Why do you think this would be valuable? What problem does it solve or what opportunity does it open?
+      placeholder: This would be valuable because...
+    validations:
+      required: false

.github/DISCUSSION_TEMPLATE/q-and-a.yml

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+title: "[Q&A] "
+labels: []
+body:
+  - type: markdown
+    attributes:
+      value: |
+        Ask a question about simulation theory, the content in this repository, or anything related you'd like help understanding.
+  - type: textarea
+    id: question
+    attributes:
+      label: Question
+      description: What would you like to know?
+      placeholder: Ask your question here...
+    validations:
+      required: true
+  - type: textarea
+    id: context
+    attributes:
+      label: Additional Context
+      description: Provide any additional context or background information that may help answer your question.
+      placeholder: Any relevant context, references, or prior research...
+    validations:
+      required: false

.github/DISCUSSION_TEMPLATE/show-and-tell.yml

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+title: "[Show and Tell] "
+labels: []
+body:
+  - type: markdown
+    attributes:
+      value: |
+        Share something you've built, discovered, or created related to simulation theory. Show the community your work!
+  - type: textarea
+    id: showcase
+    attributes:
+      label: What are you sharing?
+      description: Describe what you'd like to show the community.
+      placeholder: I'd like to share...
+    validations:
+      required: true
+  - type: textarea
+    id: links
+    attributes:
+      label: Links & Resources
+      description: Add any relevant links, demos, papers, or resources.
+      placeholder: https://...
+    validations:
+      required: false