Add FailMap release-over-release drilldown examples to docs

[Jasvina]

Problem

FailMap already supports compare and trend flows, but the repo-level public story would be stronger with clearer drilldown examples for release-over-release analysis.

Suggested scope

• improve docs or examples for compare / compare-summary / trend flows
• show how recurring signatures should be interpreted across snapshots
• keep examples aligned with the existing JSON artifact story

Acceptance criteria

• docs are clearer for first-time GitHub visitors
• commands remain runnable
• examples stay consistent with current CLI behavior