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/variables/variable-selection-options.md

@@ -0,0 +1,28 @@
++++
+title = "Enter variable Selection Options"
+type = "docs"
+[menu.docs]
+weight = 800
++++
+
+# Enter variable selection options
+
+**Selection Options** are a feature you can use to manage variable option selections. All selection options are optional, and they are off by default.
+
+## Multi-value
+
+If you turn this on, then the variable dropdown list allows users to select multiple options at the same time. For more information, refer to [Formatting multi-value variables]({{< relref "formatting-multi-value-variables.md" >}}).
+
+## Include All option
+
+Grafana adds an `All` option to the variable dropdown list. If a user selects this option, then all variable options are selected.
+
+## Custom all value
+
+This option is only visible if the **Include All option** is selected.
+
+Enter regex, globs, or lucene syntax in the **Custom all value** field to define the value of the `All` option.
+
+By default the `All` value includes all options in combined expression. This can become very long and can have performance problems. Sometimes it can be better to specify a custom all value, like a wildcard regex. 
+
+In order to have custom regex, globs, or lucene syntax in the **Custom all value** option, it is never escaped so you will have to think about what is a valid value for your data source.