docs: document logging configuration

[robfrank]

Why

Logging is currently documented only indirectly. The observability page covers the JSON console format and correlation IDs, and the settings reference lists server.logFormat, server.logIncludeTrace and server.logsDirectory, but nothing explains the configuration underneath them: where arcadedb-log.properties is looked up, how per-package levels work, how the log directory is resolved, or how an embedding application routes the engine's logs into its own backend.

What

A new Logging page under How-To > Operations (src/main/asciidoc/how-to/operations/logging.adoc), covering:

• Config file resolution. The three-step order DefaultLogger actually uses: the java.util.logging.config.file system property (which server.sh anchors to $ARCADEDB_HOME/config/), then a classpath arcadedb-log.properties, then config/arcadedb-log.properties. Plus what the shipped file configures, including the 10 x 100 MB rotation bound.
• Per-package verbosity. Including the gotcha that raising a logger to FINE does nothing while the handler stays at INFO, and the Ratis noise suppression already present in the shipped file as a worked example.
• Log directory. ${arcadedb.server.logsDirectory} placeholder resolution, the ARCADEDB_LOG_DIR environment variable forwarded by server.sh/server.bat, and why this matters on a read-only root filesystem (Kubernetes readOnlyRootFilesystem: true).
• Console format. Cross-referenced to the observability page rather than duplicating the JSON field list, plus arcadedb.installCustomFormatter=false for operators who want the properties file to be the only authority.
• SLF4J routing when embedding. arcadedb.log.impl=slf4j: bindings for Logback and Log4j2, level mapping, unchanged logger names, the namespaced MDC correlation keys, and the caveat that the SLF4J logger never reads arcadedb-log.properties.
• Custom Logger implementations.

Also cross-links from the observability logging section, and registers the page in scripts/generate-nav.py so it appears in the Antora sidebar.

Depends on ArcadeData/arcadedb#5361

The Routing through SLF4J when embedding section documents arcadedb.log.impl and Slf4jLogger, which are added by ArcadeData/arcadedb#5361 and are not yet merged. Everything else on the page describes current released behavior.

Since this repo deploys to docs.arcadedb.com on every push to main, please merge #5361 first. If it slips or changes shape, I am happy to split that section out and land the rest now.

The version marker on that section reads _(Available since v26.8.1)_, matching the current main snapshot. Worth a sanity check against the release this actually ships in.

Verification

• python docs-validator.py -- filenames, anchors and cross-references all pass. The orphaned-pages warning is pre-existing (40 files, unrelated to this change).
• mvn generate-resources -- single-page HTML builds clean and the page renders in full.
• bash scripts/migrate.sh && npm run build -- Antora build produces build/site/arcadedb/how-to/operations/logging.html, nav entry lands at nav.adoc:92, zero error-level log entries.

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