Split and clean up Metrics page

What does this MR do?

Begin shifting information around so the front page of the Metrics directory (https://docs.gitlab.com/ee/operations/metrics/) is now an introduction into the default metrics dashboard. Split alerting information out into a new page (operations/metrics/alerts.md) to keep the index page focused. Create new dashboard image to serve as a basic intro to the dashboard setup.

The metrics/dashboards/index.md page (https://docs.gitlab.com/ee/operations/metrics/dashboards/index.html) is still a mess with this MR, but I need this work to land first, and once it lands, turning the Dashboards page into the "custom dashboards" page becomes a LOT easier.

This work will be of intense interest to @nadia_sotnikova, @kbychu, @dhershkovitch, @ClemMakesApps, and @sarahwaldner.

Related issues

Related to #227441 (closed) where the IA was articulated. Related to #229593 (closed) which is a followup to this MR.

Author's checklist (required)

• Follow the Documentation Guidelines and Style Guide.
• If you have developer access or higher (for example, GitLab team members or Core Team members)
  • Apply the documentation label, plus:
    • The corresponding DevOps stage and group label, if applicable.
    • development guidelines when changing docs under doc/development/*, CONTRIBUTING.md, or README.md.
    • development guidelines and Documentation guidelines when changing docs under development/documentation/*.
    • development guidelines and Description templates (.gitlab/*) when creating/updating issue and MR description templates.
  • Assign the designated Technical Writer.

Do not add the feature, frontend, backend, ~"bug", or database labels if you are only updating documentation. These labels will cause the MR to be added to code verification QA issues.

When applicable:

• Update the permissions table.
• Link docs to and from the higher-level index page, plus other related docs where helpful.
• Add GitLab's version history note(s).
• Add the product tier badge.
• Add/update the feature flag section.
• If you're changing document headings, search doc/*, app/views/*, and ee/app/views/* for old headings replacing with the new ones to avoid broken anchors.

Review checklist

All reviewers can help ensure accuracy, clarity, completeness, and adherence to the Documentation Guidelines and Style Guide.

1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

2. Technical Writer

• Technical writer review. If not requested for this MR, must be scheduled post-merge. To request for this MR, assign the writer listed for the applicable DevOps stage.
  • Ensure Technical Writing, documentation, and a docs:: scoped label are added.
  • Add docs-only when the only files changed are under doc/*.
  • Add twdoing when starting work on the MR.
  • Add twfinished if Technical Writing team work on the MR is complete but it remains open.

3. Maintainer

1. Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
2. Ensure a release milestone is set.
3. If there has not been a technical writer review, create an issue for one using the Doc Review template.