✨(helm) allow to load custom theme file in a configMap

In order to load a custom theme file with our helm chart, we allow to
load the content of a file into a config map and then use this configmap
as a volume in the backend deployment

src/helm/impress/templates/theme_customization_file_cm.yaml

@@ -0,0 +1,10 @@
+{{- if .Values.backend.themeCustomization.enabled }}
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: docs-theme-customization
+  namespace: {{ .Release.Namespace }}
+data:
+  default.json: |
+    {{ .Values.backend.themeCustomization.file_content | toJson | indent 4 }}
+{{- end }}