docs: update TSDB storage comparison

[JasonColapietro]

Summary

• replace both obsolete per-series-file descriptions of Prometheus storage
• describe the in-memory head and WAL separately from persisted block directories
• link to the maintained on-disk layout documentation

Testing

• focused assertions for removal of the stale claims and presence of the current layout terms
• git diff --check

Site lint/build could not start locally because dependencies were absent and npm registry resolution was blocked; this draft relies on hosted CI for those gates.

Closes #1031.