Docs: Refactor Variables page and improve content (#108346)

docs/sources/dashboards/variables/add-template-variables/index.md

@@ -35,6 +35,7 @@ keywords:
   - nested
   - chained
   - linked
+  - best practices
 labels:
   products:
     - cloud
@@ -136,6 +137,13 @@ To create a variable, follow these steps:
 
 <!-- vale Grafana.Spelling = YES -->
 
+### Variable best practices
+
+- Variable drop-down lists are displayed in the order in which they're listed in the **Variables** in dashboard settings, so put the variables that you will change often at the top, so they will be shown first (far left on the dashboard).
+- By default, variables don't have a default value. This means that the topmost value in the drop-down list is always preselected. If you want to pre-populate a variable with an empty value, you can use the following workaround in the variable settings:
+  1. Select the **Include All Option** checkbox.
+  2. In the **Custom all value** field, enter a value like `+`.
+
 ## Add a query variable
 
 Query variables enable you to write a data source query that can return a list of metric names, tag values, or keys. For example, a query variable might return a list of server names, sensor IDs, or data centers. The variable values change as they dynamically fetch options with a data source query.