Add issue templates

.github/ISSUE_TEMPLATE/01-bug_report.yml

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+name: Bug Report
+description: If something isn't working as expected
+labels: [bug]
+body:
+  - type: markdown
+    attributes:
+      value: |
+        Before opening an issue, make sure to check if the issue was not previously reported or fixed.
+  - type: textarea
+    attributes:
+      label: Steps to reproduce the problem
+      description: What were you trying to do?
+      value: |
+        1.
+        2.
+        3.
+        ...
+    validations:
+      required: true
+  - type: textarea
+    attributes:
+      label: Expected behaviour
+      description: What should have happened?
+    validations:
+      required: true
+  - type: textarea
+    attributes:
+      label: Actual behaviour
+      description: What happened?
+    validations:
+      required: true
+  - type: textarea
+    attributes:
+      label: Detailed description
+    validations:
+      required: false
+  - type: textarea
+    attributes:
+      label: Specifications
+      description: |
+        If you host Retrospring, what version/commit hash of Retrospring did this issue occur in?
+
+        If a front-end issue, what browser and operating systems were you using?
+      placeholder: |
+        Retrospring 1970.0101.0
+        Ruby 3.1.3
+        Node.js 18.13.0
+
+        Google Chrome 112
+        Firefox 109.0
+        Internet Explorer 11
+
+        etc...
+    validations:
+      required: true

.github/ISSUE_TEMPLATE/02-feature_request.yml

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+name: Feature Request
+description: If you have a suggestion
+labels: [suggestion]
+body:
+  - type: markdown
+    attributes:
+      value: |
+        Please use a concise and distinct title for the issue.
+  - type: textarea
+    attributes:
+      label: Pitch
+      description: Describe your idea for a feature. Make sure it has not already been suggested/implemented/turned down before.
+    validations:
+      required: true
+  - type: textarea
+    attributes:
+      label: Motivation
+      description: Why do you think this feature is needed? Who would benefit from it?
+    validations:
+      required: true

.github/ISSUE_TEMPLATE/config.yml

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+blank_issues_enabled: false
+contact_links:
+  - name: GitHub Discussions
+    url: https://github.com/retrospring/retrospring/discussions
+    about: Please ask and answer questions here.