First draft of PR template (#4278)

* Create pull_request_template.md

* Update .github/pull_request_template.md

* Update .github/pull_request_template.md

Co-authored-by: Colleen McGinnis <[email protected]>

* blank commit so I'm the last committer

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Co-authored-by: Colleen McGinnis <[email protected]>

.github/pull_request_template.md

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+## Description
+<!-- Add a description here -->
+
+### Documentation sets edited in this PR
+
+_Check all that apply._
+
+- [ ] Stateful (`docs/en/observability/*`)
+- [ ] Serverless (`docs/en/serverless/*`)
+- [ ] Integrations Developer Guide (`docs/en/integrations/*`)
+- [ ] None of the above
+
+### Related issue
+Closes # <!-- Add the issue this PR closes here -->
+
+## Checklist
+
+<!--
+Add labels to:
+1. Backport to other versions (`backport-*`):
+    - `backport-8.x` to backport to the latest minor
+    - `backport-skip` to not backport (for example, for serverless docs) 
+    - `backport-main` to "backport" to `main` if the target branch is _not_ `main`
+    - Individual `backport-*` labels to target specific minor versions
+2. Surface blocking reviews (`needs-*-review`):
+    - `needs-writer-review` for codeowners
+    - `needs-dev-review` for dev team
+    - `needs-product-review` for PM review
+3. Build serverless preview docs: `ci:doc-build`
+-->
+
+- [ ] Product/Engineering Review
+- [ ] Writer Review
+
+### Follow-up tasks
+<!-- If you are updating the Integrations Developer Guide, you can delete this section -->
+
+_Select one._
+
+* This PR does _not_ need to be ported to another doc set because:
+  - [ ] The concepts in this PR only apply to one doc set (serverless _or_ stateful)
+  - [ ] The PR contains edits to both doc sets (serverless _and_ stateful)
+* This PR needs to be ported to another doc set:
+  - [ ] Port to stateful docs: \<link to PR or tracking issue>
+  - [ ] Port to serverless docs: \<link to PR or tracking issue>