[Documentation:System] Clarify adduser_course.py usage

[ojasvagr123]

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The Course Creation documentation shows an outdated example using null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.