Added instructor information to XLSheet and available courses API response for Generate Student List Tab

[vikrantwiz02]

This pull request enhances the handling and display of course instructor information in the academic information API. The main changes ensure that instructor names are included in both the XLS export and the available courses API response, improving clarity for end users.

Instructor information improvements:

• In the generate_xlsheet_api function (views.py), added logic to fetch and display the course instructor's name in the generated XLS sheet, showing "TBA" when no instructor is assigned. The instructor's name now appears in a dedicated row, and the list type row is shifted down accordingly.
• In the available_courses API endpoint (views.py), updated the response to include the instructor's name for each course, defaulting to "TBA" if not assigned.

Spreadsheet formatting adjustments:

• Adjusted the header row index in the XLS export to accommodate the new instructor row, ensuring headers remain correctly aligned.
• Minor cleanup by removing a redundant comment regarding column widths in the XLS export logic.