chore: update PR template to reinforce changelog (#988)

# What does this PR do?

- Added a checklist item in the PR template to ensure significant
changes are documented in the changelog.
- Updated `CHANGELOG.md` with a placeholder for version `0.2.0`.
- This is an effort to resurrect the consistent usage of the changelog
file.

Signed-off-by: Sébastien Han <seb@redhat.com>

## Test Plan

Please describe:
 - tests you ran to verify your changes with result summaries.
 - provide instructions so it can be reproduced.


## Sources

Please link relevant resources if necessary.


## Before submitting

- [ ] This PR fixes a typo or improves the docs (you can dismiss the
other checks if that's the case).
- [ ] Ran pre-commit to handle lint / formatting issues.
- [ ] Read the [contributor
guideline](https://github.com/meta-llama/llama-stack/blob/main/CONTRIBUTING.md),
      Pull Request section?
- [ ] Updated relevant documentation.
- [ ] Wrote necessary unit or integration tests.

Signed-off-by: Sébastien Han <seb@redhat.com>

.github/PULL_REQUEST_TEMPLATE.md

@@ -25,3 +25,4 @@ Please link relevant resources if necessary.
       Pull Request section?
 - [ ] Updated relevant documentation.
 - [ ] Wrote necessary unit or integration tests.
+- [ ] Added a [Changelog](https://github.com/meta-llama/llama-stack/blob/main/CHANGELOG.md) entry if the change is significant (new feature, breaking change etc.).

CHANGELOG.md

@@ -1,5 +1,14 @@
 # Changelog
 
+## 0.2.0
+
+### Added
+
+### Changed
+
+### Removed
+
+
 ## 0.0.53
 
 ### Added