Update Memory group's direction to reposition it as an Application Performance group

Why is this change being made?

Group and category renaming issue: #13714 (closed)

Key updates in this MR:

• Update vision, mission and direction of Memory Group in preparation to reposition it as an Application Performance Group
• Rename Memory Group to Application Performance Group on the Team's handbook pages

Rationale

One of the key themes of the Enablement Section is to offer a consistently great user experience regardless of location or scale. GitLab is increasingly being identified as a business-critical application with attendant requirements. It is therefore important for GitLab to be a consistently reliable, performant service for all users to fulfill its potential.

The feedback around performance has been very broad and has made it challenging for Product teams to directly address users' concerns. We need a team to fully focus on performance at all levels above infrastructure. The Memory Group has been running multiple such performance efforts on the application level for a while now, so we have decided to expand its scope and reposition it as an Application Performance Group.

What is not addressed in this MR?

Everything else!

• Updating labels, category name, the name of the group in various other places and more
• Directory restructure
• Changing file names
• Updating the other pages of the Group with more recent or relevant information (e.g. the How we work or the Team Approach and Common Themes pages)

This is only about the vision, mission and the direction of the Memory Group, which are the important parts to think about and discuss.

While doing so, I have also updated the name in few other internal pages which may cause a confusion, but the only important page is our Category Strategy page (direction/memory/) and the top section of our Team Page (handbook/engineering/development/enablement/data_stores/memory/) where we reiterate our vision and mission.

Once we are satisfied with our vision, mission and direction, we'll take care of the rest on a dedicated issue - #13714 (closed)

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.
• 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
• 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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