Docs: Move alert rule section to alerting fundamentals section (#49657)

* initial commit

* Added links to alert rules, and fixes one broken alerting relref.

docs/sources/alerting/fundamentals/alert-rules/_index.md

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+title = "Alert rules"
+description = "About Grafana alert rules"
+keywords = ["grafana", "alerting", "rules"]
+weight = 101
++++
+
+# About alert rules
+
+An alerting rule is a set of evaluation criteria that determines whether an alert instance will fire. The rule consists of one or more queries and expressions, a condition, the frequency of evaluation, and optionally, the duration over which the condition is met.
+
+While queries and expressions select the data set to evaluate, a condition sets the threshold that an alert must meet or exceed to create an alert.
+
+An interval specifies how frequently an alerting rule is evaluated. Duration, when configured, indicates how long a condition must be met. The alert rules can also define alerting behavior in the absence of data.
+
+- [Alert rule types]({{< relref "./alert-rule-types.md" >}})
+- [Alert instances]({{< relref "./alert-instances.md" >}})
+- [Organising alert rules]({{< relref "./organising-alerts.md" >}})
+- [Annotation and labels]({{< relref "../annotation-label/_index.md" >}})

docs/sources/alerting/fundamentals/alert-rules/alert-instances.md

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+title = "Alert instances"
+description = "Learn about Grafana alert instances"
+keywords = ["grafana", "alerting", "instances"]
+weight = 103
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+
+# Alert instances
+
+Grafana managed alerts support multi-dimensional alerting. Each alert rule can create multiple alert instances. This is exceptionally powerful if you are observing multiple series in a single expression.
+
+Consider the following PromQL expression:
+
+```promql
+sum by(cpu) (
+  rate(node_cpu_seconds_total{mode!="idle"}[1m])
+)
+```
+
+A rule using this expression will create as many alert instances as the amount of CPUs we are observing after the first evaluation, allowing a single rule to report the status of each CPU.
+
+{{< figure src="/static/img/docs/alerting/unified/multi-dimensional-alert.png" caption="A multi-dimensional Grafana managed alert rule" >}}

docs/sources/alerting/fundamentals/alert-rules/alert-rule-types.md

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+title = "Alert rule types"
+description = "Learn about the different alert rule types"
+keywords = ["grafana", "alerting", "rule types"]
+weight = 102
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+
+# Alert rule types
+
+Grafana supports several alert rule types, the following sections will explain their merits and demerits and help you choose the right alert type for your use case.
+
+## Grafana managed rules
+
+Grafana-managed rules are the most flexible alert rule type. They allow you to create alerts that can act on data from any of your existing data sources.
+
+In additional to supporting any datasource you can also add additional [expressions]({{< relref "../../panels/query-a-data-source/use-expressions-to-manipulate-data/_index.md" >}}) to transform your data and express alert conditions.
+
+## Mimir, Loki and Cortex rules
+
+To create Mimir, Loki or Cortex alerts you must have a compatible Prometheus data source. You can check if your data source is compatible by testing the data source and checking the details if the ruler API is supported.
+
+{{< figure src="/static/img/docs/alerting/unified/mimir-datasource-check.png" caption="Successfully connected to a Mimir Prometheus datasource" max-width="40%" >}}
+
+## Recording rules
+
+Recording rules are only available for compatible Prometheus data sources like Mimir, Loki and Cortex.
+
+A recording rule allows you to save an expression's result to a new set of time series. This is useful if you want to run alerts on aggregated data or if you have dashboards that query the same expression repeatedly.
+
+Read more about [recording rules](https://prometheus.io/docs/prometheus/latest/configuration/recording_rules/) in Prometheus.

docs/sources/alerting/fundamentals/alert-rules/organising-alerts.md

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+title = "Organising alert rules"
+description = "Learn how to organize alert rules"
+keywords = ["grafana", "alerting", "organization"]
+weight = 104
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+
+## Namespaces and groups
+
+Alerts can be organized using Folders for Grafana-managed rules and namespaces for Mimir or Loki rules and group names.
+
+### Namespaces
+
+When creating Grafana-managed rules, the folder can be used to perform access control and grant or deny access to all rules within a specific folder.
+
+### Groups
+
+All rules within a group are evaluated at the same **interval**.
+
+Alert rules and recording rules within a group will always be evaluated **sequentially**, meaning no rules will be evaluated at the same time and in order of appearance.
+
+> **Note** If you want rules to be evaluated concurrently and with different intervals, consider storing them in different groups.