Docs: Add variable topics - custom, constant, text box (#26471)

* moved some content into other topics

* Update variable-value-tags.md

* Update variable-selection-options.md

* Update add-query-variable.md

* Update templates-and-variables.md

* Create add-custom-variable.md

* Update menu.yaml

* Update templates-and-variables.md

* Update templates-and-variables.md

* editing content

* Create add-text-box-variable.md

* added links

* content updates

* update-content

* Update templates-and-variables.md

* Update menu.yaml

* Update add-constant-variable.md

* Update add-constant-variable.md

* Update docs/sources/variables/add-query-variable.md

Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>

* Update docs/sources/variables/variable-value-tags.md

Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>

* Update docs/sources/variables/variable-value-tags.md

Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>

* Update docs/sources/variables/variable-value-tags.md

Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>

Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>

docs/sources/menu.yaml

@@ -254,7 +254,17 @@
     - link: /variables/templates-and-variables/
       name: Overview
     - link: /variables/add-query-variable/
-      name: Add a query variable
+      name: Add query variable
+    - link: /variables/add-custom-variable/
+      name: Add custom variable
+    - link: /variables/add-text-box-variable/
+      name: Add text box variable
+    - link: /variables/add-constant-variable/
+      name: Add constant variable
+    - name: Selection options
+      link: /variables/variable-selection-options/
+    - name: Value groups/tags
+      link: /variables/variable-value-tags/
     - link: /variables/global-variables/
       name: Global variables
     - link: /variables/url-variables/