Move Scalability direction into new template and update links

Why is this change being made?

This change is to move the scalability direction from the Handbook into the direction section of about.gitlab.com/direction where all the rest of the product direction lives.

As part of the work for this MR I have:

• Gotten high level feedback on adapting the direction to a product direction template in a google doc
• Created a new page for the direction
• Updated the link in the SaaS platforms Direction page
• Updated the Scalability team page to point to the new direction page

https://scalability-direction-move.about.gitlab-review.app/direction/saas-platforms/scalability/

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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