chore(github): clean up PR template (#2077)

Makes the PR template look nicer with comments to assist PR creators

## Checklist
- [x] Breaking changes are clearly marked as such in the PR description
and changelog
- [x] New behavior is reflected in tests
- [x] [The specification](https://github.com/FuelLabs/fuel-specs/)
matches the implemented behavior (link update PR if changes are needed)

### Before requesting review
- [x] I have reviewed the code myself
- [x] I have created follow-up issues caused by this PR and linked them
here

### After merging, notify other teams

[Add or remove entries as needed]

- [ ] [Rust SDK](https://github.com/FuelLabs/fuels-rs/)
- [ ] [Sway compiler](https://github.com/FuelLabs/sway/)
- [ ] [Platform
documentation](https://github.com/FuelLabs/devrel-requests/issues/new?assignees=&labels=new+request&projects=&template=NEW-REQUEST.yml&title=%5BRequest%5D%3A+)
(for out-of-organization contributors, the person merging the PR will do
this)
- [ ] Someone else?

.github/pull_request_template.md

@@ -1,6 +1,8 @@
-[Link to related issue(s) here, if any]
+## Linked Issues/PRs
+<!-- List of related issues/PRs -->
 
-[Short description of the changes.]
+## Description
+<!-- List of detailed changes -->
 
 ## Checklist
 - [ ] Breaking changes are clearly marked as such in the PR description and changelog