Docs: Update _index.md (#29546)

docs/sources/variables/variable-types/_index.md

 +++
-title = "Variables types"
+title = "Add variables"
 weight = 140
 +++
 
@@ -17,4 +17,4 @@ Grafana uses several types of variables.
 | Interval   | Interval variables represent time spans. [Add an interval variable]({{< relref "add-interval-variable.md" >}}).   |
 | Ad hoc filters   | Key/value filters that are automatically added to all metric queries for a data source (InfluxDB, Prometheus, and Elasticsearch only). [Add ad hoc filters]({{< relref "add-ad-hoc-filters.md" >}}).   |
 | Global variables   | Built-in variables that can be used in expressions in the query editor. Refer to [Global variables]({{< relref "global-variables" >}}).   |
-| Chained variables   | Variable queries can contain other variables. Refer to [Chained variables]({{< relref "chained-variables.md" >}}).   |
\ No newline at end of file
+| Chained variables   | Variable queries can contain other variables. Refer to [Chained variables]({{< relref "chained-variables.md" >}}).   |