Update Ecosystem Stage pages in the Engineering handbook

Why is this change being made?

In anticipation of Ecosystem becoming a Stage, !86129 (merged), we:

• move the Engineering handbook pages for ecosystem from dev/create/ecosystem to dev/ecosystem/
• Change the Ecosystem Backend pages to be for the Integrations team (dev/ecosystem/ecosystem-be to dev/ecosystem/integrations)
• Create new pages (a little empty right now, let's iterate!) for Contributor Experience and Foundations.
• Correct links and remove unnecessary mentions in the Create Stage pages
• Remove Staff Engineer Gearing exception, because we have three separate teams now

Approval

• @dsatcher
• @timzallmann

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.

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