Improve documentation for group_type and group_id values (#571)

**Description**
This PR improves the documentation for:
  - `group_type` in `harbor_group`
  - `group_id` in `harbor_project_member_group`

**Changes**

- Replaced the previous hardcoded note (`group type 3 is OIDC group`)
with a complete mapping of all valid numeric values.
- Clearly documented the allowed values:
  - `1` = `ldap`
  - `2` = `internal`
  - `3` = `oidc`

**Why**
The previous documentation only mentioned that value `3` corresponds to
OIDC, which was incomplete and potentially confusing.

---------

Signed-off-by: Yann ILAS <yann.ilas@gmail.com>

Author: yilas

docs/resources/group.md

@@ -25,7 +25,10 @@ resource "harbor_group" "storage-group" {
 ### Required
 
 - `group_name` (String) The name of the group.
-- `group_type` (Number) 3. Note: group type 3 is OIDC group.
+- `group_type` (Number) The numeric identifier of the group type. Valid values are `1`, `2`, or `3` :
+  - `1` = `ldap`
+  - `2` = `internal`
+  - `3` = `oidc`
 
 ### Optional
 

docs/resources/project_member_group.md

@@ -38,7 +38,10 @@ resource "harbor_project_member_group" "main" {
 
 ### Optional
 
-- `group_id` (Number) 3. Note: group type 3 is OIDC group.
+- `group_id` (Number) The numeric identifier of the group type. Valid values are `1`, `2`, or `3` :
+  - `1` = `ldap`
+  - `2` = `internal`
+  - `3` = `oidc`
 - `group_name` (String) The name of the group member entity.
 - `ldap_group_dn` (String) The distinguished name of the group within AD/LDAP.