Prometheus tuning and Metric Thinning (#565)

Restructures the p8s-jaeger-grafana page into a header page with dedicated
pages for each of the three components.

Add new content about prom tuning / metric thinning for prometheus.

kiali/kiali#5151

content/en/docs/Configuration/p8s-jaeger-grafana/_index.md

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+---
+title: "Prometheus, Jaeger, Grafana"
+description: "Kiali data sources and add-ons."
+---
+
+Prometheus is a required telemetry data source for Kiali. Jaeger is a highly recommended tracing data source. Kiali also offers a simple Grafana add-on integration. This page describes how to configure Kiali to communicate with these dependencies.
+
+Read the dedicated configuration page to learn more.

content/en/docs/Configuration/p8s-jaeger-grafana/grafana.md

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+---
+title: "Grafana"
+description: >
+  This page describes how to configure Grafana for Kiali.
+---
+
+## Grafana configuration
+
+Istio provides [preconfigured Grafana
+dashboards](https://istio.io/latest/docs/ops/integrations/grafana/) for the
+most relevant metrics of the mesh. Although Kiali offers similar views in its
+metrics dashboards, it is not in Kiali's goals to provide the advanced querying
+options, nor the highly customizable settings, that are available in Grafana.
+Thus, it is recommended that you use Grafana if you need those advanced
+options.
+
+Kiali can provide a direct link from its metric dashboards to the equivalent or
+most similar Grafana dashboard, which is convenient if you need the powerful
+Grafana options. For these links to appear in Kiali you need to manually
+configure the Grafana URL, like in the following example:
+
+```yaml
+spec:
+  external_services:
+    grafana:
+      enabled: true
+      # Grafana service name is "grafana" and is in the "telemetry" namespace.
+      in_cluster_url: 'http://grafana.telemetry:3000/'
+      # Public facing URL of Grafana
+      url: 'http://my-ingress-host/grafana'
+```
+

content/en/docs/Configuration/p8s-jaeger-grafana/jaeger.md

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+---
+title: "Jaeger"
+description: >
+  This page describes how to configure Jaeger for Kiali.
+---
+
+
+## Jaeger configuration
+
+Jaeger is a _highly recommended_ service because [Kiali uses distributed
+tracing data for several features]({{< relref "../../Features/tracing" >}}),
+providing an enhanced experience.
+
+By default, Kiali will try to reach Jaeger at the GRPC-enabled URL of the form
+`http://tracing.<istio_namespace_name>:16685/jaeger`, which is the usual case
+if you are using [the Jaeger Istio
+add-on](https://istio.io/latest/docs/ops/integrations/jaeger/#option-1-quick-start).
+If this endpoint is unreachable, Kiali will disable features that use
+distributed tracing data.
+
+If your Jaeger instance has a different service name or is installed to a
+different namespace, you must manually provide the endpoint where it is
+available, like in the following example:
+
+```yaml
+spec:
+  external_services:
+    tracing:
+      # Enabled by default. Kiali will anyway fallback to disabled if
+      # Jaeger is unreachable.
+      enabled: true
+      # Jaeger service name is "tracing" and is in the "telemetry" namespace.
+      # Make sure the URL you provide corresponds to the non-GRPC enabled endpoint
+      # if you set "use_grpc" to false.
+      in_cluster_url: 'http://tracing.telemetry:16685/jaeger'
+      use_grpc: true
+      # Public facing URL of Jaeger
+      url: 'http://my-jaeger-host/jaeger'
+```
+
+Minimally, you must provide `spec.external_services.tracing.in_cluster_url` to
+enable Kiali features that use distributed tracing data. However, Kiali can
+provide contextual links that users can use to jump to the Jaeger console to
+inspect tracing data more in depth. For these links to be available you need to
+set the `spec.external_services.tracing.url` which may mean that you should
+expose Jaeger outside the cluster.
+