Move cross-functional to a top level

Why is this change being made?

Initially the new Cross-functional Prioritization Content was added as a child page of our Product Development Flow. Since these two are separate, we are moving the prioritization pages to a top level folder.

Discussion at https://gitlab.slack.com/archives/C03AWM7780G/p1655405407874569?thread_ts=1655403370.731799&cid=C03AWM7780G

Tasks left

• Update remaining references to /cross-functional-prioritization.html and /cross-functional-dashboard-reviews.html (in Product, QA, UX pages)
• Create redirects for
  1. /product-development-flow/cross-functional-prioritization.html to /cross-functional-prioritization
  2. /product-development-flow/cross-functional-prioritization/cross-functional-dashboard-reviews.html to cross-functional-prioritization/cross-functional-dashboard-reviews.html

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
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• 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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