fix(docs): Fix Thanos sidecar migration doc

[boernd]

Since Thanos version v0.37.0 the documentation is broken due to thanos-io/thanos#7820. Update the documentation accordingly.

What this PR does

Updates the "Migrate from Thanos to Mimir using Thanos sidecar" documentation.

---

Note

Adds NGINX config and notes to spoof Prometheus /metrics (including prometheus_tsdb_lowest_timestamp_seconds) for Thanos sidecar >= v0.37.0.

• Docs (Mimir migration via Thanos sidecar):
  • Add technical note that Thanos sidecar (v0.37.0+) reads prometheus_tsdb_lowest_timestamp_seconds from /metrics.
  • NGINX config:
    • New location /prometheus/metrics returning a gzipped text payload with prometheus_tsdb_lowest_timestamp_seconds 0.
    • Integrate this block in both example and full Mimir distributed NGINX configs.
  • Update explanation to describe the third spoofed endpoint (/metrics) and its purpose.

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