[fractal3000]

Russian version of #243: the same rework of the Observability section for Jmix 3, targeting release_3_ru.

• features/observability.adoc becomes an overview; details move to new pages under features/observability/: Metrics, Tracing, Logging, Profiling, Performance Diagnostics;
• app-properties and ui-properties describe the four observation properties;
• What's New 3.0 links to the new pages and properties;
• the observability-ex1 example project holds the code included in the pages, with tests;
• the navigation lists the new pages and the monitoring guide.

The structure matches the English version: the same anchors, cross-references, figures, code includes and table rows. The guides stay in English, as on the rest of the Russian site.

Images are kept to one per page (six in total). The text is self-contained: every fact shown on a figure is also stated in the text, and the example traces are given as text span trees, so readers and AI agents get the full content without images. The removed figures are deleted in separate commits; please squash-merge so that they do not reach the history of the target branch.

Merge order is the same as for #243: the navigation links to the monitoring guide from jmix-framework/jmix-observability-tracing-sample#2.

See: #242