docs: migrate monitor label actions and welcome copy from trunk-io/docs

[mintlify]

Summary

Migrates the most recent doc changes from trunk-io/docs (the legacy GitBook source) into this Mintlify docs repo.

Changes

• Welcome page (PR #663) — Simplifies the description of how Trunk detects flakes (drop stack-trace-embedding implementation detail, replace with "surface those differences for quick triage").
• Monitor label actions (PR #656) — Documents the new Apply labels action on Pass-on-Retry, Failure Rate, and Failure Count monitors:
  • Reframes monitors around a configurable action (Classify test status vs. Apply labels) instead of a fixed detection type.
  • Adds an Action section to each monitor page.
  • Adds a "Dry-Running with Labels" section on the Flake Detection overview.
  • Replaces the "Coming soon" stub on Test Labels with a full "Automatic labeling from monitors" section.

Context

Other recent trunk-io/docs PRs (#657, #652, #650, #648) were already migrated in earlier syncs; this PR catches up the two remaining ones.