[MR] Aug 23 - Iterable Updates to email nurture handbook

Why is this change being made?

💡 Provide a detailed answer to the question on why this change is being proposed, in accordance with our value of Transparency.

Updating handbook to reflect the following changes:

• Move from Mailgun to Iterable
• Links to epics, internal docs
• Figma files (moved from Mural for this issue)

Author and Reviewer Checklist

Please verify the check list and ensure to tick them off before the MR is merged.

• 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
  • Updating since we moved all emails from Mailgun to Iterable, updating with relevant documentation, and Figma files (moved from Mural)
• 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
• For transparency, share this MR with the audience that will be impacted.

  • Team: @dambrold @p_cordero @bstallings

    • For high-priority company-wide announcements work with the internal communications team to post the update in #company-fyi and align on a plan to circulate in additional channels like the "While You Were Iterating" Newsletter

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