Update Environments Error Budget DRI docs

Why is this change being made?

Over the last few weeks, I was a DRI on some Error Budget investigation issues. I spent some time trying to understand the root cause and propose solutions. Later, I received feedback

Red means that we reach out the infra teams and ask them for support. You are not expected to know how to track the root cause down on your own. It’s more a learning and cross-company collaboration opportunity

Knowing this beforehand can save an engineer time if they are asked to be a DRI on the issue, so I want to add this to the handbook so everyone can benefit from the feedback.

Reviewers

I'm hoping the rest of the team has some experience and changes they'd like to add to this section. Please add your feedback.

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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