Change organizational structure for product process section of handbook

Why is this change being made?

I noticed that (a) there were some pages that were no longer relevant and (b) we didn't have a great place to document operating cadences. I moved some pages around, deleted a couple unnecessary pages, and merged content into more logical places. Here's a list of the changes:

1. Create "operating cadences" folder, under the "product process" folder
2. Move "people group for Product Management" up a level to the "product process" folder
3. Move content from the "Product Management Procedures" page to the "operating cadences" and the "How to Engage with Product Management" pages, depending on the nature of the content.
4. Move "board meeting prep" pages under the "operating cadences" folder
5. Delete the "product operations procedures" folder (which is now empty, given above changes)
6. Delete "product management operations" folder (which is now empty, given above changes)

This MR does not include any changes to the existing content, only where the content is recorded within the handbook.

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
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• 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: For changes that affect your direct team, share in your group Slack channel
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Commits

• Update 8 files

• /content/handbook/product/product-processes/product-mgt-operations/pm-operating-procedures/_index.md
• /content/handbook/product/product-processes/product-mgt-operations/pm-operating-procedures/people-ops-for-product.md
• /content/handbook/product/product-processes/product-mgt-operations/_index.md
• /content/handbook/product/product-processes/product-mgt-operations/board-deck-prep.md
• /content/handbook/product/product-processes/operating-cadences/_index.md
• /content/handbook/product/product-processes/operating-cadences/board-deck-prep.md
• /content/handbook/product/product-processes/people-ops-for-product.md
• /content/handbook/product/how-to-engage.md

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