[Documentation:System] Clarify adduser_course.py usage (#732)

Closes Submitty/Submitty#10341

**Why is this Change Important & Necessary?**

The Course Creation documentation shows an outdated example using `null`
with `adduser_course.py` and does not explain how to add non-instructor
users using the `--user_group` option. Since the script now supports an
optional numeric `REGISTRATION_SECTION` and a `--user_group` argument
(default `1`, Instructor), the documentation should reflect the current
implementation to avoid confusion for system administrators.

**What is the New Behavior?**

The Course Creation page now:

- Uses the current `--user_group` syntax instead of `null`.
- Documents the general command format for `adduser_course.py`.
- Explains the `user_group` roles (Instructor, Full/ Limited Grader,
Student).
- Clarifies that `REGISTRATION_SECTION` is optional and must be numeric.

**What steps should a reviewer take to verify this change?**

1. Open the Course Creation documentation page.
2. Locate the section “Add existing instructor(s) to the course
database.”
3. Confirm the updated example, command format, and role descriptions.

**Automated Testing & Documentation**

This is a documentation-only change. No automated testing is required.

**Other information**

This is my first contribution to Submitty. I reviewed the current
`sbin/adduser_course.py` implementation to ensure the documentation
accurately reflects its behavior.

Co-authored-by: Barb Cutler <bmcutler@users.noreply.github.com>

Author: ojasvagr123

_docs/sysadmin/configuration/course_creation.md

@@ -154,11 +154,27 @@ you can follow these [instructions](ansible_course_creation) to create a course
    For more information: [Database Overview](/sysadmin/troubleshooting/database_overview)
 
 
-3. Add existing instructor(s) to the course database:
+3. Add existing instructor(s) to the course database, and optionally add other 
+   types of users using the `adduser_course.py` helper script.
+   To add an existing instructor:
 
    ```
-   sudo /usr/local/submitty/sbin/adduser_course.py <USERNAME> <SEMESTER> <COURSE> null
+   sudo /usr/local/submitty/sbin/adduser_course.py <INSTRUCTOR_USERNAME> <SEMESTER> <COURSE> --user_group 1
    ```
+   The general command format is:
+
+   ```
+   sudo /usr/local/submitty/sbin/adduser_course.py <USER_ID> <SEMESTER> <COURSE> [REGISTRATION_SECTION] [--user_group GROUP]
+   ```
+
+   - `REGISTRATION_SECTION` is optional and must be numeric.
+
+   - `--user_group` defines the user's role in the course and defaults to `1` (Instructor) if omitted.
+
+     - `1` – Instructor  
+     - `2` – Full Access Grader  
+     - `3` – Limited Access Grader  
+     - `4` – Student  
 
 
 4. Create registration section(s):