Virtual Events Handbook Updates - 20200414 to 20200417 - V1.0

Why is this change being made?

Virtual events handbook updates happening between April 14, 2020 - April 17, 2020, 7 pm PT for Marketing Agility project: Virtual event excellence.

List of changes:

• Refine different types of GitLab Hosted Virtual Events on the handbook and update terminology everywhere for consistency.
• Add links to Marketo program templates for sponsored virtual events.

related to : https://gitlab.com/gitlab-com/marketing/marketing-agility/-/issues/2

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• Did you assign this change to the correct DRI of the page or information you are changing?

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