Rewrite Scalability direction page post reorg

Why is this change being made?

This change is updating the scalability direction page to be reflective of our renewed 3-5 year strategy and group responsibility. This closes #12865 (closed) (FY25 - Scalability Direction Update (Product#12865 - closed))

Preview available at - https://scalability-direction-update.about.gitlab-review.app/direction/saas-platforms/scalability/

This MR is a result of the direction conversations that we have been conducting as a group. We started by collaborating on a Figma board and iterated to create 3-5 year goals that were proposed to Marin and Fabian. The agenda from that meeting is available here.

The direction page is being updated to include these goals, but to summarize, they are:

1. Observability across the Production Fleet is accessible for all.
2. Availability metrics better reflect the user experience.
3. Paved roads are the default for all team members
4. Solutions at GitLab Follow the Well Architected Services Framework
5. Engineering Portal is the home for engineers at GitLab

This MR will be followed by the handbook page updates for each team, to look at how we will achieve these goals.

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
  • 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
• 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
  • Department: If the update affects your department, share the MR in your department Slack channel
  • Company: If the update affects all (or the majority of) GitLab team members, post an update in #whats-happening-at-gitlab linking to this MR
    • 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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