Adopt new GitHub Issues Forms syntax

Author: Kyle-Ye

.github/ISSUE_TEMPLATE/BUG_REPORT.md

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+name: Bug Report
+description: Something isn't working as expected
+labels: [bug]
+body:
+- type: markdown
+  attributes:
+    value: "## Thank you for contributing to Swift-DocC!"
+- type: markdown
+  attributes:
+    value: |
+      Before you submit your issue, please complete each textarea
+      below with the relevant details for your bug, and complete
+      the steps in the checklist
+- type: textarea
+  attributes:
+    label: Description
+    description: |
+      A short description of the incorrect behavior. 
+      If you think this issue has been recently introduced and did not occur in an 
+      earlier version, please note that. If possible, include the last version that 
+      the behavior was correct in addition to your current version.
+  validations:
+    required: true
+- type: checkboxes
+  attributes:
+    label: Checklist
+    options:
+    - label: If possible, I've reproduced the issue using the `main` branch of this package.
+      required: false
+    - label: This issue hasn't been addressed in an [existing GitHub issue](https://github.com/apple/swift-docc/issues).
+      required: true
+- type: textarea
+  attributes:
+    label: Expected Behavior
+    description: Describe what you expected to happen.
+  validations:
+    required: false
+- type: textarea
+  attributes:
+    label: Actual behavior
+    description: Describe or copy/paste the behavior you observe.
+  validations:
+    required: false
+- type: textarea
+  attributes:
+    label: Steps To Reproduce
+    description: |
+      Explanation of how to reproduce the incorrect behavior. 
+      This could include a reduced version of your documentation bundle,
+      or a link to the documentation (or code) that is exhibiting the issue.
+    placeholder: |
+      1. In this environment...
+      2. With this config...
+      3. Run '...'
+      4. See error...
+  validations:
+    required: false
+- type: textarea
+  attributes:
+    label: Swift-DocC Version Information
+    description: |
+      examples:
+        - **Swift-DocC version:** `5.6.0` for example, or a commit hash.
+        - **Swift Compiler version:** Output from `swiftc --version`.
+    value: |
+        - Swift-DocC version:
+        - Swift Compiler version:
+    render: markdown
+  validations:
+    required: false

.github/ISSUE_TEMPLATE/BUG_REPORT.yml

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+name: Feature Request
+description: A suggestion for a new feature
+labels: [enhancement]
+body:
+- type: markdown
+  attributes:
+    value: "## Thank you for contributing to Swift-DocC!"
+- type: markdown
+  attributes:
+    value: |
+      Before you submit your issue, please replace each textarea
+      below with information about your proposed feature.
+- type: markdown
+  attributes:
+    value: "## Feature Request"
+- type: input
+  attributes:
+    label: Feature Name
+  validations:
+    required: true
+- type: textarea
+  attributes:
+    label: Description
+    description: |
+      A description of your proposed feature.
+      Sample documentation catalogs that show what's missing, 
+      or what new capabilities will be possible, are very helpful! 
+      Provide links to existing issues or external references/discussions, if appropriate.
+  validations:
+    required: true
+- type: textarea
+  attributes:
+    label: Motivation
+    description: A description of the use-case this proposal is trying to serve.
+  validations:
+    required: false
+- type: textarea
+  attributes:
+    label: Importance
+    description: |
+      A description of the importance of this change.
+      Does this feature unlock entirely new use-cases?
+      Or is it possible to achieve the same functionality with existing functionality
+      and this change would improve the user experience?
+  validations:
+    required: false
+- type: textarea
+  attributes:
+    label: Alternatives Considered
+    description: If you considered alternative approaches for this feature, please include them here.
+  validations:
+    required: false