Document JSON measurements

JSON measurements let you capture structured objects and arrays such as device state, config snapshots, and diagnostics, and now have their own documentation page.

• Added a JSON measurement guide covering how values are captured, how the type is detected, and the validators it supports.
• Added a live preview that renders a JSON payload the way it appears in the dashboard.
• Clarified that numeric matrices stay multi-dimensional while objects and other arrays become JSON measurements.