Add database maintenance example (#824)

Adding an example override file to scale pods to 0 which would try to connect to the databases during database maintenance operations, ex. MVU, and provide an easy place for customers to operate the migrator commands.

Customers can use this override file with the `sourcegraph` Helm chart, on top of their own override file(s), when needing to do things like let the Postgres pods upgrade to PG v16 on SG v6, before upgrading to SG v7

### Checklist

- [x] Follow the [manual testing
process](https://github.com/sourcegraph/deploy-sourcegraph-helm/blob/main/TEST.md)
- [ ] Update
[changelog](https://github.com/sourcegraph/deploy-sourcegraph-helm/blob/main/charts/sourcegraph/CHANGELOG.md)
- [ ] Update [Kubernetes update
doc](https://docs.sourcegraph.com/admin/updates/kubernetes)

### Test plan

- Tested on test instance
- Have used with customers in the past

<!--
As part of SOC2/GN-104 and SOC2/GN-105 requirements, all pull requests
are REQUIRED to
provide a "test plan". A test plan is a loose explanation of what you
have done or
implemented to test this, as outlined in our Testing principles and
guidelines:

https://docs.sourcegraph.com/dev/background-information/testing_principles
  Write your test plan here after the "Test plan" header.
-->

Author: marcleblanc2

charts/sourcegraph/examples/database-maintenance/database-maintenance-override.yaml

@@ -0,0 +1,99 @@
+# Override file for database maintenance
+
+################################################################################
+# Purpose
+################################################################################
+
+# Scales down all pods which would usually connect to the database
+# Leaves databases and other pods unchanged
+# Allows you to operate the Migrator commands, if needed, on the migrator init container in the frontend pod
+
+################################################################################
+# Usage
+################################################################################
+
+# 1) Append this additional override file to your usual Helm command, and run it
+
+# helm upgrade --install \
+#   sourcegraph \
+#   sourcegraph/sourcegraph \
+#   --namespace $NAMESPACE \
+#   --values instances/staging/override.yaml \
+#   --values examples/database-maintenance/database-maintenance-override.yaml
+
+# 2) Get logs from the migrator init container within the frontend pod
+# Note: Be sure you're seeing the logs from the most recent replicaSet,
+# especially if you have to redeploy this a few times
+
+# kubectl -n $NAMESPACE logs -c migrator \
+#   $(kubectl -n $NAMESPACE get pods --sort-by=.metadata.creationTimestamp -o name | grep frontend | tail -1)
+
+# 3) When finished, re-run your usual Helm command without this additional override file,
+# to scale pods back to their usual replicaCount
+
+# helm upgrade --install \
+#   sourcegraph \
+#   sourcegraph/sourcegraph \
+#   --namespace $NAMESPACE \
+#   --values instances/staging/override.yaml
+
+################################################################################
+# Migrator
+# If needed, operate the migrator commands using the args here
+# See available commands at
+# https://sourcegraph.com/docs/admin/updates/migrator/migrator-operations#commands
+# See details about Multi-version upgrades at
+# https://sourcegraph.com/docs/admin/deploy/kubernetes#multi-version-upgrades
+################################################################################
+
+frontend:
+  # Need one frontend pod to run the migrator as an init container
+  # replicaCount: 1
+  # Or, leave the frontend pod and migrator disabled to let the database pods startup on their own
+  replicaCount: 0
+
+# migrator:
+
+  ### Drift check
+  # args:
+  # - drift
+  # - --version=v6.12.5040
+  # - --schema=frontend
+  # - --schema=codeinsights
+  # - --schema=codeintel
+
+  ### Multi-version upgrade
+  # args:
+  # - upgrade
+  # - --from=v5.1.6
+  # - --to=v6.12.5040
+
+################################################################################
+# Databases
+# Do not modify the databases in this file, unless needed
+################################################################################
+
+# codeInsightsDB:
+
+# codeIntelDB:
+
+# pgsql:
+
+################################################################################
+# Scale down all other pods which would usually connect to the database
+################################################################################
+
+gitserver:
+  replicaCount: 0
+
+indexedSearch:
+  replicaCount: 0
+
+preciseCodeIntel:
+  replicaCount: 0
+
+searcher:
+  replicaCount: 0
+
+worker:
+  replicaCount: 0