docs: add gitops_command_prefix to settings ref

Document gitops_command_prefix in the Repository settings API
reference.

The field is already supported and covered in the GitOps guide, but
it was missing from the settings reference that claims to describe
all available fields. Add the reference entry and cross-link back to
the behavior guide.

Signed-off-by: Chmouel Boudjnah <chmouel@redhat.com>

Author: chmouel

docs/content/docs/api/settings.md

@@ -32,6 +32,19 @@ settings:
 
 {{< /param >}}
 
+{{< param name="gitops_command_prefix" type="string" id="param-gitops-command-prefix" >}}
+Sets a custom prefix for GitOps commands such as `/test`, `/retest`, and `/cancel`.
+Use a plain word such as `pac`; Pipelines-as-Code adds the leading slash
+automatically. For command behavior and examples, see the
+[GitOps commands guide]({{< relref "/docs/guides/gitops-commands/advanced" >}}).
+
+```yaml
+settings:
+  gitops_command_prefix: "pac"
+```
+
+{{< /param >}}
+
 {{< param name="policy" type="Policy" >}}
 Defines authorization policies for the repository. These policies control which users can trigger PipelineRuns under different conditions.