Add co-hosting event guidance to handbook

[Dahianna]

Please create a page to add the co-hosting events guidance to the handbook. You can find the content here: https://docs.google.com/document/d/1PzGc-fK5AefbMH9dVjigUwQaibz3tk4VAA-4KoGTqFQ/edit?usp=sharing Thanks!