Helm: Draft release notes for 5.6 (#10233)

* Helm: Draft release notes for  5.6

* tacole02 feedback

docs/sources/helm-charts/mimir-distributed/release-notes/v5.6.md

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+---
+title: "Grafana Mimir Helm chart version 5.6 release notes"
+menuTitle: "V5.6 release notes"
+description: "Release notes for Grafana Mimir Helm chart version 5.6"
+weight: 300
+---
+
+# Grafana Mimir Helm chart version 5.6 release notes
+
+Grafana Labs is excited to announce version 5.6 of the Grafana Mimir Helm chart, which is compatible with Grafana Mimir v2.15 and Grafana Enterprise Metrics (GEM) v2.15. The `mimir-distributed` Helm chart is the best way to install Mimir on Kubernetes.
+
+The highlights that follow include the top features, enhancements, and bug fixes in this release. For a comprehensive list of changes, see the [Helm chart changelog](https://github.com/grafana/mimir/tree/main/operations/helm/charts/mimir-distributed/CHANGELOG.md).
+
+## Important changes
+
+The rollout-operator version has been updated to version 0.20.0.
+
+The `track_sizes` feature for Memcached pods has been removed.
+
+## Features and enhancements
+
+Support has been added for `federation-frontend` in the values file. The federation-frontend allows you to query data from multiple GEM clusters using a single endpoint. For more information about cluster query federation, refer to the [federation-frontend documentation](https://grafana.com/docs/enterprise-metrics/<GEM_VERSION>/operations/cluster-query-federation).
+
+You can create a service account for the alertmanager with dedicated permissions and authorizations by setting `alertmanager.serviceAcount.create` in the values file.