[fractal3000]

Reworks the Observability section for Jmix 3 (English version).

features/observability.adoc becomes an overview: signals, Jmix-specific observations, how signals link to each other, production principles. Details move to new pages under features/observability/:

• Metrics: Jmix timers and their tags, the legacy UI metrics schema, Prometheus and OTLP export, the Jmix 3 dashboards, alerts;
• Tracing: Jmix spans and attributes, sampling, context propagation, spans of business operations;
• Logging: levels and categories, MDC and jmixUser, trace IDs in log lines, SQL output, centralized collection;
• Profiling: the Pyroscope agent, reading a Vaadin request profile, linking profiles to spans;
• Performance Diagnostics: slow views, data access (N+1), remote calls, memory including a leak example, locks.

Other changes:

• app-properties and ui-properties describe jmix.core.data-observation-enabled, jmix.core.use-user-info-for-observation, jmix.ui.ui-observation-enabled and jmix.ui.legacy-monitoring-enabled;
• 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.

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: the navigation and the pages link to the monitoring guide added in jmix-framework/jmix-observability-tracing-sample#2, and that guide links back to these pages. The two PRs should be merged close together. jmix-framework/jmix-observability-logging-sample#6 and jmix-framework/jmix-application-logging-sample#4 link to the new Logging page and go after this PR.

The Russian version will be a separate PR to release_3_ru.

See: #242