Add knowledge sharing Deep Dive guidelines to handbook

Other teams have expressed an interest in running knowledge sharing sessions, as originally described here: https://gitlab.com/gitlab-org/manage/issues/9

Therefore, I am moving the details to the handbook for wider visibility.

https://add-knowledge-sharing-201s.about-src.gitlab.com/handbook/communication/#knowledge-sharing
https://add-knowledge-sharing-201s.about-src.gitlab.com/handbook/communication/knowledge-sharing/

Note: We initially called these sessions 201s, but have decided Deep Dives is a clearer term.

Feedback on the location of this information in the handbook is welcome!

Thanks to @jamedjo for running the first 201 session and adding the preparation guidelines.