docs: explain the P2P diagnostic workflow

Guide users through Diagnostic summary, Diag logging, transfer progress,
symptom checks, and evidence collection from both devices. Link the guide
from the P2P, troubleshooting, reporting, and documentation entry points.
Keep transport compatibility and observation limits explicit.

Validated with the troubleshooting documentation Inspector, 145 local
references, 34 heading anchors, and current English UI labels. Runtime
source and translations are unchanged.

docs/README.md

@@ -14,7 +14,7 @@ A document's original date and reviewed-version fields describe its baseline. Re
 | Find hosting services proposed by the community | [Third-party hosting listings](../README.md#hosting-services-proposed-by-the-community) |
 | Set up S3-compatible storage | [Object Storage Setup](setup_object_storage.md) and [transfers and service costs](setup_object_storage_costs.md) |
 | Set up peer-to-peer synchronisation | [P2P communication model](p2p.md), then [Peer-to-Peer Setup](setup_p2p.md) |
-| Diagnose P2P connectivity | [Browser-assisted check](../src/apps/webpeer/README.md#p2p-connection-check), [network tips](tips/p2p-sync-tips.md), and [self-hosted TURN](../docker/coturn/README.md) |
+| Diagnose P2P connectivity or transfer activity | [Diagnostic summary and logs](tips/p2p-diagnostics.md), [browser-assisted check](../src/apps/webpeer/README.md#p2p-connection-check), [network tips](tips/p2p-sync-tips.md), and [self-hosted TURN](../docker/coturn/README.md) |
 | Understand a setting or command | [Settings reference](settings.md) |
 | Read or customise synchronisation status | [Status indicators and CSS snippets](tips/status-display.md) |
 | Investigate or recover from a problem | [Troubleshooting](troubleshooting.md), then the relevant [recovery procedure](recovery.md) |