docs: add v26.38.0 notes for dictionary compression and observability

Documents three user-facing additions to the v26.38.0 release notes:

* **Dictionary compression**, as a public-preview feature section. Covers the
  memory it saves when a column repeats values, the `EXPERIMENTAL ARRANGEMENT
  COMPRESSION` opt-in, and the CPU-for-memory tradeoff plus the replica
  replacement that changing the option triggers. The option is per cluster
  only; it is rejected on unmanaged clusters, per
  test/sqllogictest/managed_cluster.slt:450.

* **Observability integrations for Self-Managed**, covering export of metrics
  and optionally logs to Datadog, Honeycomb, Google Cloud Monitoring,
  Prometheus remote write, or any OTLP endpoint, with the bundled monitoring
  stack the Terraform modules install as the fallback for deployments that
  have no stack of their own.

* **materialize-debug-freshness**, the agent skill added in
  MaterializeInc/agent-skills#35.

All 16 links and their anchors were verified to resolve against a local Hugo
build, which passes clean.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: maheshwarip

doc/user/content/releases/_index.md

@@ -23,11 +23,69 @@ both Cloud and Self-Managed. See [Release schedule](/releases/schedule) for deta
 *Released to Materialize Cloud: 2026-08-19* <br>
 *Released to Materialize Self-Managed: 2026-08-20* <br>
 
+### Dictionary compression {#v26.38-dictionary-compression}
+
+{{< public-preview />}}
+
+Dictionary compression reduces the memory that
+[arrangements](/get-started/arrangements/#arrangements) use when a column holds the same values repeatedly. Instead of storing a repeated column value each time it appears, Materialize stores that value once and has each row reference it. This can reduce steady state memory requirements after [hydration](/concepts/hydration/) has completed.
+
+Dictionary compression is off by default. You opt in per cluster with the
+`EXPERIMENTAL ARRANGEMENT COMPRESSION` option:
+
+```mzsql
+-- Turn compression on for a new cluster
+CREATE CLUSTER my_cluster (
+    SIZE = '100cc',
+    EXPERIMENTAL ARRANGEMENT COMPRESSION = true
+);
+
+-- Or turn it on for an existing cluster
+ALTER CLUSTER my_cluster SET (EXPERIMENTAL ARRANGEMENT COMPRESSION = true);
+```
+
+For more information, see:
+- [Guide: Dictionary compression](/transform-data/dictionary-compression/), including [when it helps and when it does not](/transform-data/dictionary-compression/#the-tradeoff)
+- [`CREATE CLUSTER`: Dictionary compression](/sql/create-cluster/#dictionary-compression)
+- [`ALTER CLUSTER`: Dictionary compression](/sql/alter-cluster/#dictionary-compression)
+
+### Integrate with your observability stack {#v26.38-self-managed-observability}
+
+<red>*Materialize Self-Managed only*</red>
+
+Materialize Self-Managed now integrates with the observability tools you already
+run. You can export metrics, and optionally logs, from Materialize to Datadog,
+Honeycomb, Google Cloud Monitoring, Prometheus remote write, or any monitoring
+backend with an Open Telemetry (OTLP) endpoint. Template dashboards and alerts are provided to
+help you get started.
+
+Follow the instructions for your destination:
+- [Datadog](/manage/monitor/self-managed/datadog/)
+- [Honeycomb](/manage/monitor/self-managed/honeycomb/)
+- [Google Cloud Monitoring](/manage/monitor/self-managed/google-cloud-monitoring/)
+- [Prometheus remote write](/manage/monitor/self-managed/prometheus-remote-write/), for Mimir, Amazon Managed Prometheus, or Grafana Cloud
+- [OpenTelemetry](/manage/monitor/self-managed/opentelemetry/), for any other OTLP endpoint, including your own collector
+
+If you don't have an observability stack set up, the [Materialize Terraform
+modules](/self-managed-deployments/installation/#install-using-terraform-modules)
+can deploy one alongside Materialize. It collects metrics from Materialize and
+from your Kubernetes cluster, collects Materialize's container logs and
+Kubernetes events, stores both in your own object storage, and ships [Grafana](/manage/monitor/self-managed/grafana/)
+dashboards and Alertmanager alert rules to query them. The stack is controlled by
+the `enable_observability` variable, which defaults to `true` starting with
+v11.0.0 of the modules.
+
+For more information, see:
+- [Monitoring Self-Managed Materialize](/manage/monitor/self-managed/)
+- [How logs and metrics are stored and delivered](/manage/monitor/self-managed/storage/)
+- [Alerting](/manage/monitor/self-managed/alerting/)
+
 ### Improvements {#v26.38-improvements}
 - **Notice for single-replica sources on multi-replica clusters**: Materialize now warns when a command leaves a cluster holding more than one replica alongside PostgreSQL, MySQL, or SQL Server sources, which always run on a single replica, since the extra replicas make those sources neither more fault tolerant nor faster to ingest.
 - **Self-Managed: Graceful resizing of system clusters**: `ALTER CLUSTER ... SET (SIZE ...)` on a system cluster such as `mz_catalog_server` or `mz_system` now runs as a background graceful reconfiguration, with a 24-hour default deadline and a rollback on timeout, instead of recreating the whole replica set at once, so `SHOW CLUSTERS` settles on the new configuration rather than flipping to it.
 
 ### Agent Skills {#v26.38-agent-skills}
+- **materialize-debug-freshness**: New agent skill for diagnosing why an object is behind wall-clock time, whether that surfaces as a stale materialized view, index, or sink, or as a freshness alert. Running on the read-only tools of the Materialize developer MCP server, it ranks what is lagging, attributes the lag to a single hop, then rules out in-progress hydration, a replica dominated by one dataflow, and per-worker skew before naming the culprit operator and the SQL responsible for the expensive work.
 - **mz-deploy**: New agent skill covering the `mz-deploy` CLI — project layout, the compile/test/apply/stage/promote workflow, deploy IDs and staging suffixes, schema-granularity conflict detection, stable API schemas, profile resolution, and the `EXECUTE UNIT TEST` grammar.
 - **mz-sql-lsp**: New Claude Code plugin, installable from the `agent-skills` repository's new `materialize` plugin marketplace, that registers the `mz-deploy` language server for `.sql` files so agents can use go-to-definition, hover, and workspace symbols in an mz-deploy project instead of text search.