This commit is in response to issue json-schema-org#658. Here, the following changes were done:

    - Added 'required' keyword to all code samples in the 'Define properties' section
    - Reworded and restructured the 'Define properties' section for better understanding, and removed the 'Add the properties object' sub-section because it was unnecessary.
    - Added an infobox to give more information about optional properties

Author: chidiadi01