docs(getting-started): replace invented scan example with real output

[cicdteam]

Summary

The "First scan" example in docs/getting-started.md invented column names (CLASS / FIX) that don't exist in the terminal output and used lowercase severities. Replaces the block with the actual remetric scan output captured against the e2e demo stack, including:

• Phase progress tracker (▶ cardinality... done (120ms) etc.).
• Real box-drawing severity table with the actual columns (SEVERITY / METRIC / LABEL / SERIES / UNIQUE / EST. REDUCTION).
• The per-finding detail block for the CRITICAL row, including the paste-ready metric_relabel_configs snippet and the Reference: link to the docs page (the link populated by PR feat(docs): findings reference site at remetric.dev + DocURL in every finding #13's DocURL wiring).

Rewrites the "Reading the report" section to describe the three layers actually visible (phase tracker, severity table, per-finding detail), with column meanings.

Test plan

• mkdocs build --strict clean.
• CI green.
• Post-merge: docs.yml redeploys; https://remetric.dev/getting-started/#first-scan shows the new example.