docs: Update SAST docs documenting `SAST_ANALYZER_IMAGE_TAG`

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/232660

doc/user/application_security/sast/index.md

@@ -473,6 +473,7 @@ The following are Docker image-related CI/CD variables.
 |---------------------------|---------------------------------------------------------------------------------------------------------------------------------------|
 | `SECURE_ANALYZERS_PREFIX` | Override the name of the Docker registry providing the default images (proxy). Read more about [customizing analyzers](analyzers.md). |
 | `SAST_EXCLUDED_ANALYZERS` | Names of default images that should never run. Read more about [customizing analyzers](analyzers.md).                                 |
+| `SAST_ANALYZER_IMAGE_TAG` | Override the default version of analyzer image. Read more about [pinning the analyzer image version](#pinning-to-minor-image-version).                                 |
 
 #### Vulnerability filters