Write an article explaining how the fallback configuration feature works

[czeslavo]

The article / docs updates should cover:

• FallbackConfiguration feature gate and its default mode of operation (excluding broken objects)
• --use-last-valid-config-for-fallback CLI flag (backfilling broken objects from the last valid cache state)
• new diagnostics server endpoints
• new Kubernetes events
• new Prometheus metrics

[czeslavo]

PRs in the docs repo that have to be merged before closing this issue:

• docs(kic): fallback configuration guide docs.konghq.com#7463
• docs(kic): update observability references for fallback config feature docs.konghq.com#7467