Alert Management detail view docs

What does this MR do?

Adds documentation for the Alert Management detail view.

Note: This feature is currently behind the alert_management_detail feature flag.

Related issues

Issue: #213911 (closed)

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.

When applicable:

• [n/a] Update the permissions table.
• [n/a] Link docs to and from the higher-level index page, plus other related docs where helpful.
• [n/a] Add GitLab's version history note(s).
• [n/a] Add the product tier badge.
• [n/a] Add/update the feature flag section.
• [n/a] 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.

☝ I n/a'd these because it was taken care of when the list docs were added - !31225 (merged). This MR just adds a new header + screenshot below but permissions / versions should be the same.

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

• Optional: 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.
  • Add Technical Writing and docs:: workflow label.
  • Add docs-only when the only files changed are under doc/*.

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.