Add BrightTalk replay instructions and multi-webcast series set-up to Webcasts page

Why is this change being made?

• Detailed instructions for scheduling a replay (mock-live webcast) in BrightTalk were not currently available in the handbook. This MR adds those instructions.

• Adds instructions for a multi-webcast series set-up (Zoom).

• Moves the Webcasts handbook page from the revenue marketing handbook to the marketing level. This is because multiple groups within GitLab Marketing run Virtual Events, and this change moves the logistical information to a higher level. The Revenue Marketing team has also changed to Integrated Marketing.

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
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added.
• Assign reviewers for this change to the correct DRI(s)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the "Maintained by" section in on the page being edited.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies.
• If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR.
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

---