Add an Alerting overview page and reconcile alerting docs with the UI

[CandiceChislett]

Adds a landing page for the Alerting section, which previously had none. It lists every alerting page with what it is for, promotes the rules vs detectors explanation out of rules.md, and documents the controls shared across the section once, instead of repeating them on each page.

Reconciles Status, Alert Rules and Service Anomaly Detectors against the current UI:

• Status: the needs-attention band wording and its two states, state chips only appearing for states that are present, and cards holding rules that need attention being highlighted and promoted to the front
• Alert Rules: renamed to match the UI, four state counters rather than five, sortable column headers, Labels and the Notifies empty state in the expanded view, and the real namespace and group filters
• Service Anomaly Detectors: detector badge states and the last evaluation placeholder for paused services

Also switches British spellings to American per the style guide, and replaces the Status screenshot.