feat(docs): Migrate to mdbook

docs/src/common-library/howto/helm-templates.md

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+# Helm templates
+
+Some fields in the common library `values.yaml` allow the use of Helm templates for their values.
+This is often indicated by a remark similar to `Helm template enabled` in the field description.
+
+This feature allows you to have the value of that key set to the output of the given Helm template.
+
+## Example:
+
+Given the following `values.yaml`
+
+```yaml
+image:
+  repository: k8s.gcr.io/git-sync/git-sync
+  tag: v3.6.2
+
+additionalContainers:
+  subcleaner:
+    name: subcleaner
+    image: |-
+      {{ printf "%s:%s" .Values.image.repository (default .Chart.AppVersion .Values.image.tag) | quote }}
+    args:
+      - --repo=https://github.com/KBlixt/subcleaner.git
+      - --branch=master
+      - --depth=1
+      - --root=/add-ons/subcleaner
+```
+
+This would render as follows:
+
+```yaml
+image:
+  repository: k8s.gcr.io/git-sync/git-sync
+  tag: v3.6.2
+
+additionalContainers:
+  subcleaner:
+    name: subcleaner
+    image: k8s.gcr.io/git-sync/git-sync:v3.6.2
+    args:
+      - --repo=https://github.com/KBlixt/subcleaner.git
+      - --branch=master
+      - --depth=1
+      - --root=/add-ons/subcleaner
+```