Add issue templates for bug reports, documentation improvements, and feature requests

Author: edwardchalstrey1

.github/ISSUE_TEMPLATE/bug_report.yml

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+name: Bug Report
+description: File a bug report.
+title: "[Bug]: "
+labels: ["bug"]
+body:
+  - type: markdown
+    attributes:
+      value: |
+        Thanks for taking the time to fill out this bug report!
+  - type: dropdown
+    id: software-component
+    attributes:
+      label: Where did you find this bug?
+      multiple: true
+      options:
+        - PyGambit
+        - Gambit CLI
+        - Gambit GUI
+        - Other
+    validations:
+      required: true
+  - type: textarea
+    id: what-happened
+    attributes:
+      label: What happened?
+      description: Also tell us, what did you expect to happen?
+      placeholder: Tell us what you see and provide a screenshot if appropriate.
+      value: "A bug happened!"
+    validations:
+      required: true

.github/ISSUE_TEMPLATE/documentation_improvement.yml

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+name: Documentation Improvement
+description: Suggest a change to the Gambit documentation.
+title: "[Documentation]: "
+labels: ["documentation"]
+body:
+  - type: markdown
+    attributes:
+      value: |
+        Thanks for taking the time to fill out this documentation improvement request!
+  - type: textarea
+    id: request-details
+    attributes:
+      label: Requested documentation improvement
+      description: Please provide details about the documentation improvement you would like to see. If you want to edit a specific page, please include the URL. If you have an idea for a new page, please explain where it should fit in the documentation structure.
+      placeholder: Describe the documentation improvement in detail.
+      value: "A documentation improvement suggestion!"
+    validations:
+      required: true

.github/ISSUE_TEMPLATE/feature_request.yml

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+name: Feature Request
+description: Suggest a new feature or improvement to the Gambit software.
+labels: ["enhancement"]
+body:
+  - type: markdown
+    attributes:
+      value: |
+        Thanks for taking the time to fill out this feature request!
+  - type: dropdown
+    id: software-component
+    attributes:
+      label: This feature request is for...
+      multiple: true
+      options:
+        - PyGambit
+        - Gambit CLI
+        - Gambit GUI
+        - Other
+  - type: textarea
+    id: request-details
+    attributes:
+      label: Requested enahancement or feature
+      description: Please provide details about the feature or improvement you would like to see and add any relevant labels.
+      placeholder: Describe the feature or improvement in detail.
+      value: "A feature or improvement suggestion!"
+    validations:
+      required: true