Update Assembly writeup on current Assembly page

Why is this change being made?

We've updated the process, content, timeline, and additional information on GitLab Assembly on the People Communications and team member engagement page and will use that as the SSOT for Assembly. https://about.gitlab.com/handbook/people-group/employment-branding/people-communications/#gitlab-assembly

Updating the previous Assembly page to be more accurately titled "GitLab All-Company Meetings" since it covers both Assembly and the Quarterly Kickoff. Removed outdated content from the Assembly section, updated the Assembly section to align with the new updates on the People Comms & team member engagement page, and added a new timing section at the top to set the stage for the content below.

Author Checklist

• Provided a concise title for this Merge Request (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, and the content is SAFE
• Assign reviewers for this MR to the correct Directly Responsible Individual/s (DRI)
  • 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 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
  • The when to get approval handbook section explains the workflow in more detail
• 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.

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