Document descriptions of the metrics uses in the alerts and dashboard templates

[R-Lawton]

Based on the alerts and dashboards added in Kuadrant/kuadrant-operator#439,
add a new docs page that details all the relevant metrics used in queries for those alerts & dashboards.

• metric name, what it is, type of metric (counter, gauge)
• where the metric comes from (e.g. istio, istiod, envoy, gateway-api-state, kube-state, kubelet)
• links to the different collectors (kube-state, kubelet)
• explanation of how we join metrics from different jobs e.g. linking a HTTPRoute to istio requests metrics
• why certain queries were chosen