[docs] Clarify for what version the Helm chart docs are applicable (#8172)

**What this PR does / why we need it**:

The docs and reference of the Helm chart in the documentation is only
applicable for the Loki Helm chart `>= 3.0` and therefore does not apply
to the `grafana/loki-stack` chart, which depends on the Loki Helm chart
`^2.15.2`
([requirements.yaml](https://github.com/grafana/helm-charts/blob/main/charts/loki-stack/requirements.yaml#L5)).

Also added a headline to the "Install Loki using Helm" index page,
because it looks awkward without, see screenshot.


![2023-01-17_20-28-52](https://user-images.githubusercontent.com/281260/212994261-f817f556-4cbf-454f-920f-875c5c085689.png)


**Which issue(s) this PR fixes**:

Fixes: #8152

Signed-off-by: Christian Haudum <christian.haudum@gmail.com>

docs/sources/installation/helm/_index.md

@@ -12,9 +12,11 @@ keywords:
   - installation
 ---
 
+# Install Loki using Helm
+
 The [Helm](https://helm.sh/) chart allows you to configure, install, and upgrade Grafana Loki within a Kubernetes cluster.
 
-This guide contains the following sections:
+This guide references the Loki Helm chart version 3.0 or greater and contains the following sections:
 
 {{< section menuTitle="true" >}}
 

docs/sources/installation/helm/reference.md

@@ -17,6 +17,11 @@ keywords: []
 
 This is the generated reference for the Loki Helm Chart values.
 
+> **Note:** This reference is for the Loki Helm chart version 3.0 or greater.
+> If you are using the `grafana/loki-stack` Helm chart from the community repo,
+> please refer to the `values.yaml` of the respective Github repository
+> [grafana/helm-charts](https://github.com/grafana/helm-charts/tree/main/charts/loki-stack).
+
 <!-- Override default values table from helm-docs. See https://github.com/norwoodj/helm-docs/tree/master#advanced-table-rendering -->
 
 <table>

production/helm/loki/reference.md.gotmpl

@@ -17,6 +17,11 @@ keywords: []
 
 This is the generated reference for the Loki Helm Chart values.
 
+> **Note:** This reference is for the Loki Helm chart version 3.0 or greater.
+> If you are using the `grafana/loki-stack` Helm chart from the community repo,
+> please refer to the `values.yaml` of the respective Github repository
+> [grafana/helm-charts](https://github.com/grafana/helm-charts/tree/main/charts/loki-stack).
+
 <!-- Override default values table from helm-docs. See https://github.com/norwoodj/helm-docs/tree/master#advanced-table-rendering -->
 {{ define "chart.valuesTableHtml" }}
 <table>