docs: explain mesh data path to dashboard and Observatory

[NgoQuocViet2001]

Summary

• document that sensing-server is the aggregator path used by the REST API, WebSocket stream, dashboard, and Observatory
• add source-build and Docker commands for connecting ESP32 mesh data to the UI
• expand the Observatory DEMO troubleshooting notes for the common standalone-aggregator/UDP-port confusion

Problem

Issue #383 asks how to get from a working ESP32 aggregator to visualization in the sensing server and Observatory. The existing guide mentions ESP32 mesh, the sensing server, and Observatory separately, but does not state that the standalone hardware aggregator is a packet-inspection tool and that sensing-server must own the UDP listener for the UI to see live data.

Validation

• git diff --check
• Documentation-only change; no runtime code was modified.

Closes #383

[ruvnet]

Docs change is well-scoped, directly addresses the confusion in #383 (standalone hardware aggregator vs sensing-server owning the UDP listener), and the table of view URLs is exactly what was missing from the user guide. The added troubleshooting bullets at the bottom of the Observatory section are a nice touch. Merging.