Add tips to Group Conversation Handbook to encourage participation

Why is this change being made?

We noticed a slight drop in attendance during the Group Conversation on Monday 2021-08-30. We speculated that there are more meetings at the 8:00 AM PST time slot because it is a great meeting time.

A few team members also added that it might be because people feel that they need to do homework before attending or that questions need to be focused on the slide topics only.

This MR adds a section Tips that encourage participation. [Issue here.](https://gitlab.com/gitlab-com/ceo-shadow/tasks/-/issues/213

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
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