First draft of PR template

[bmorelli25]

Description

This is a first draft of PR template. The Serverless/Stateful section is a bit long. Thoughts?

[mergify]

This pull request does not have a backport label. Could you fix it @bmorelli25? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-/d./d is the label to automatically backport to the /d./d branch. /d is the digit
  NOTE: backport-skip has been added to this pull request.

[colleenmcginnis]

What do you think about something like this? This approach would:

• Prompt the author to specify which documentation set they are editing.
• Prompt the author to link to either a port PR or tracking issue rather than giving them the option to either open a PR or add a label. IMO an issue might be easier to track in our existing workflows (our GitHub board).

## 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

- [ ] Product/Engineering Review
- [ ] Writer Review

<!-- If you are updating the Integrations Developer Guide, you can delete this section -->
### Follow-up tasks

_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>

[colleenmcginnis]

✨ 🐥