Docs: Add a query variable topic (#25852)

* Update global-variables.md

* Create _index.md

* Create add-query-variable.md

* added variable task files

* Update templates-and-variables.md

* added content

* content edits

* Update formatting-multi-value-variables.md

* added content and updated menu

* Update add-query-variable.md

* applied edits

* Update docs/sources/variables/formatting-multi-value-variables.md

Co-authored-by: Marcus Efraimsson <marcus.efraimsson@gmail.com>

* Update docs/sources/variables/formatting-multi-value-variables.md

Co-authored-by: Marcus Efraimsson <marcus.efraimsson@gmail.com>

* Update docs/sources/variables/formatting-multi-value-variables.md

Co-authored-by: Marcus Efraimsson <marcus.efraimsson@gmail.com>

* applied edits

* Update docs/sources/variables/filter-variables-with-regex.md

Co-authored-by: Marcus Efraimsson <marcus.efraimsson@gmail.com>

* Update docs/sources/variables/formatting-multi-value-variables.md

Co-authored-by: Marcus Efraimsson <marcus.efraimsson@gmail.com>

* Update docs/sources/variables/formatting-multi-value-variables.md

Co-authored-by: Marcus Efraimsson <marcus.efraimsson@gmail.com>

* Update formatting-multi-value-variables.md

Co-authored-by: Marcus Efraimsson <marcus.efraimsson@gmail.com>

docs/sources/menu.yaml

@@ -249,12 +249,18 @@
   children:
     - link: /variables/templates-and-variables/
       name: Overview
+    - link: /variables/add-query-variable/
+      name: Add a query variable
     - link: /variables/global-variables/
       name: Global variables
     - link: /variables/url-variables/
       name: URL variables
     - link: /variables/advanced-variable-format-options/
       name: Advanced variable formats
+    - link: /variables/formatting-multi-value-variables/
+      name: Formatting multi-value variables
+    - link: /variables/filter-variables-with-regex/
+      name: Filter variables with regex
     - link: /variables/repeat-panels-or-rows/
       name: Repeat panels or rows
 - name: What's new in Grafana