[Improvement] Update developer guide with details for DeleteCoursesCommand and adjust layout of command class diagram

[chewycj]

Fixes #140 and fixes #163

• Changed layout of command class diagram to vertical for better readability.
• Added details for DeleteCoursesCommand into the DG, with the sequence diagram.

[hzxnancy]

The sequence diagram looks good but you may need to activate the storage class when calling the getCourse() method

[chewycj]

Thanks for catching that!! 👍 Added activation bar for Storage, how is the diagram now? @hzxnancy