Add instructions for Helm chart updates in the pull request template (#9514)

**What this PR does / why we need it**:

Add additional instructions for Helm charts in the pull request
template.

**Special notes for your reviewer**:

**Checklist**
- [ ] Reviewed the
[`CONTRIBUTING.md`](https://github.com/grafana/loki/blob/main/CONTRIBUTING.md)
guide (**required**)
- [ ] Documentation added
- [ ] Tests updated
- [ ] `CHANGELOG.md` updated
- [ ] Changes that require user attention or interaction to upgrade are
documented in `docs/sources/upgrading/_index.md`

---------

Signed-off-by: Michel Hollands <michel.hollands@grafana.com>

.github/pull_request_template.md

@@ -11,3 +11,4 @@ Fixes #<issue number>
 - [ ] Tests updated
 - [ ] `CHANGELOG.md` updated
 - [ ] Changes that require user attention or interaction to upgrade are documented in `docs/sources/upgrading/_index.md`
+- [ ] For Helm chart changes bump the Helm chart version in `production/helm/loki/Chart.yaml` and update `production/helm/loki/CHANGELOG.md` and `production/helm/loki/README.md`. [Example PR](https://github.com/grafana/loki/commit/d10549e3ece02120974929894ee333d07755d213)