docs: add default value workaround (#72545)

* Update _index.md

Fixes #https://github.com/grafana/support-escalations/issues/6376

Variables : Added Default values

* Update docs/sources/dashboards/variables/_index.md

Co-authored-by: Isabel <76437239+imatwawana@users.noreply.github.com>

* Ran prettier to resolve build errors

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Co-authored-by: Isabel <76437239+imatwawana@users.noreply.github.com>
Co-authored-by: Isabel Matwawana <isabel.matwawana@grafana.com>
Co-authored-by: Jack Baldry <jack.baldry@grafana.com>

docs/sources/dashboards/variables/_index.md

@@ -63,3 +63,6 @@ The following dashboards in Grafana Play provide examples of template variables:
 
 - Variable drop-down lists are displayed in the order they are listed in the variable list in Dashboard settings.
 - 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 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 `+`.