Remove unused sections from Engineering index page

Why is this change being made?

These sections duplicate content elsewhere:

1. Engineering Performance Reviews/Talent Assessment
   1. This said 'The above has led us to follow GitLab's Talent Assessment process from FY22 on and use that handbook page as SSOT.' so I think we can just delete it.
2. Access Requests
   1. This looks like it was added when ARs were a new process. They aren't any more.
3. Guidelines for automation and access tokens
   1. This is just a link to a sub-page; moved it to the link list at the top.
4. Error Budgets
   1. Also just a link.
5. Demos
   1. Linked to https://about.gitlab.com/handbook/engineering/development/demos.html, which is a 404! I updated the page so #demos goes to the actual demo process.
6. Visualization Tools / Monitoring Tools
   1. https://about.gitlab.com/handbook/engineering/monitoring/ covers this and is more likely to be up to date, so I just moved a couple of links there.

Part of chief-of-staff-cto#2.

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.

---