Clarify PR/Contribution guidelines and issue templates (#2281)

Co-authored-by: Dylan <dylan.hurd@openai.com>

.github/ISSUE_TEMPLATE/4-feature-request.yml

@@ -0,0 +1,31 @@
+name: 🎁 Feature Request
+description: Propose a new feature for Codex
+labels:
+  - feature
+  - needs triage
+body:
+  - type: markdown
+    attributes:
+      value: |
+        Is Codex missing a feature that you'd like to see? Feel free to propose it here.
+
+        Before you submit a feature:
+        1. Search existing issues for similar features. If you find one, 👍 it rather than opening a new one.
+        2. The Codex team will try to balance the varying needs of the community when prioritizing or rejecting new features. Not all features will be accepted. See [Contributing](https://github.com/openai/codex#contributing) for more details.
+
+  - type: textarea
+    id: feature
+    attributes:
+      label: What feature would you like to see?
+    validations:
+      required: true
+  - type: textarea
+    id: author
+    attributes:
+      label: Are you interested in implementing this feature?
+      description: Please wait for acknowledgement before implementing or opening a PR.
+  - type: textarea
+    id: notes
+    attributes:
+      label: Additional information
+      description: Is there anything else you think we should know?

.github/pull_request_template.md

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+# External (non-OpenAI) Pull Request Requirements
+
+Before opening this Pull Request, please read the "Contributing" section of the README or your PR may be closed:
+https://github.com/openai/codex#contributing
+
+If your PR conforms to our contribution guidelines, replace this text with a detailed and high quality description of your changes.