Add small section about how to prioritise reducing budget spend

Why is this change being made?

groupintegrations opened a self-discovery issue gitlab-org/gitlab#337220 (closed) with the questions we had 7 months ago when Error Budgets were first introduced.

Since that issue was open, Error Budgets, and the docs for it, have gone through a number of iterations. When I review that issue and correlate it with the current state of documentation, I can see that answers to all of our original questions can be found in the current docs, which are:

• https://about.gitlab.com/handbook/engineering/error-budgets/ (the primary page for error budges)
• https://docs.gitlab.com/ee/development/application_slis/rails_request_apdex.html (deeper information about Apdex and how to customise it)
• https://docs.gitlab.com/ee/development/stage_group_dashboards.html#error-budget (error budget dashboards)

I’ve noted in the issue gitlab-org/gitlab#338383 (comment 864399574) that I think the main handbook page could benefit from two additional sections that represent what a lot of groups will probably want to know when encountering Error Budgets for the first time.

1. Which types of errors are included?
2. How do we prioritise reducing our budget spend?

These sections are very small, and mostly redirect the reader to the location of the information in other docs.

This MR introduces 2., another MR introduces 1. !100176 (merged), to allow discussion to happen on the MR without blocking each other.

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.
• 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.

---