Add link from API to subgroups doc

doc/api/groups.md

@@ -756,7 +756,7 @@ Parameters:
 | `two_factor_grace_period`            | integer | no       | Time before Two-factor authentication is enforced (in hours). |
 | `project_creation_level`             | string  | no       | Determine if developers can create projects in the group. Can be `noone` (No one), `maintainer` (Maintainers), or `developer` (Developers + Maintainers). |
 | `auto_devops_enabled`                | boolean | no       | Default to Auto DevOps pipeline for all projects within this group. |
-| `subgroup_creation_level`            | string  | no       | Allowed to create subgroups. Can be `owner` (Owners), or `maintainer` (Maintainers). |
+| `subgroup_creation_level`            | string  | no       | Allowed to [create subgroups](../user/group/subgroups/index.md#creating-a-subgroup). Can be `owner` (Owners), or `maintainer` (Maintainers). |
 | `emails_disabled`                    | boolean | no       | Disable email notifications |
 | `avatar`                             | mixed   | no       | Image file for avatar of the group. [Introduced in GitLab 12.9](https://gitlab.com/gitlab-org/gitlab/-/issues/36681) |
 | `mentions_disabled`                  | boolean | no       | Disable the capability of a group from getting mentioned |
@@ -830,7 +830,7 @@ PUT /groups/:id
 | `two_factor_grace_period`            | integer | no       | Time before Two-factor authentication is enforced (in hours). |
 | `project_creation_level`             | string  | no       | Determine if developers can create projects in the group. Can be `noone` (No one), `maintainer` (Maintainers), or `developer` (Developers + Maintainers). |
 | `auto_devops_enabled`                | boolean | no       | Default to Auto DevOps pipeline for all projects within this group. |
-| `subgroup_creation_level`            | string  | no       | Allowed to create subgroups. Can be `owner` (Owners), or `maintainer` (Maintainers). |
+| `subgroup_creation_level`            | string  | no       | Allowed to [create subgroups](../user/group/subgroups/index.md#creating-a-subgroup). Can be `owner` (Owners), or `maintainer` (Maintainers). |
 | `emails_disabled`                    | boolean | no       | Disable email notifications |
 | `avatar`                             | mixed   | no       | Image file for avatar of the group. [Introduced in GitLab 12.9](https://gitlab.com/gitlab-org/gitlab/-/issues/36681) |
 | `mentions_disabled`                  | boolean | no       | Disable the capability of a group from getting mentioned |