[DOCS-11412] Document subtest support for Synthetic Multistep API tests (#30284)

* [DOCS-11412] Document subtest support to Synthetic Multistep API tests

* Minor syntax update

* Fix image width to match browser docs

* Update content/en/synthetics/multistep.md

Co-authored-by: domalessi <[email protected]>

* Update content/en/synthetics/multistep.md

Co-authored-by: domalessi <[email protected]>

* Update content/en/synthetics/multistep.md

Co-authored-by: domalessi <[email protected]>

* Update content/en/synthetics/multistep.md

Co-authored-by: domalessi <[email protected]>

* Update content/en/synthetics/multistep.md

Co-authored-by: domalessi <[email protected]>

---------

Co-authored-by: domalessi <[email protected]>

Author: hakanb

content/en/synthetics/multistep.md

@@ -194,6 +194,20 @@ To display your list of variables, type `{{` in your desired field.
 
 {{< img src="synthetics/api_tests/use_variable.mp4" alt="Using Variables in Multistep API tests" video="true" width="90%" >}}
 
+### Subtests
+
+Multistep API tests support subtests, allowing you to reuse existing API tests or extract steps into reusable components. You can nest subtests up to two levels deep.
+
+To use an existing API test as a subtest, click **Subtest**, go to the **From Existing Test** tab, and select an API test from the dropdown menu.
+
+To convert steps from your current API test into a subtest, click on the **Extract From Steps** tab, select the recorded steps you want to extract, and click **Convert to Subtest**. 
+
+{{< img src="synthetics/multistep_tests/subtest.png" alt="UI for adding a subtest to a Multistep API test" width="60%" >}}
+
+To override a subtest variable in a Multistep API test, define it in the parent test using the same name. A variable always uses the first value assigned to it.
+
+If you don't need to run a subtest independently, you can pause it. It still runs as part of the Multistep API test but is not executed on its own.
+
 ## Test failure
 
 A test is considered `FAILED` if a step does not satisfy one or several assertions or if a step's request prematurely failed. In some cases, the test can indeed fail without being able to test the assertions against the endpoint, these reasons include: