docs: Document database: sync and the databaseSync experimental feature

[FXschwartz]

The 4.0 changelog advertises offline-first apps via database: sync and the serverpod_offline_sync module, but docs/ has zero hits for sync, offline_sync, databaseSync, or createSyncSession, and two pages state the opposite:

• docs/06-concepts/03-data-and-the-database/02-database/01-tables.md:26-32 and docs/06-concepts/lookups/model-reference.md:17 - database "accepts three values"; the analyzer has four (server, client, all, sync) and its error text for shared packages literally says "all" or "sync" (analyzer/models/definitions.dart:561-570, restrictions.dart:245-256).
• docs/06-concepts/01-server-fundamentals/03-configuration.md:332 and docs/06-concepts/lookups/configuration-reference.md:110 - "No experimental features are available"; databaseSync exists (tools/serverpod_cli/lib/src/config/experimental_feature.dart:13-19) and is required to use database: sync (restrictions.dart:310-322).

What the generator produces with sync on: lib/src/generated/sync_tables.dart on server and client, Client.createSyncSession(path, {runMigrations, isDebugMode, persistentUserId}), and the CRDT interceptor wiring in the generated Serverpod class (generator/dart/library_generators/library_generator.dart:53-100, 1620-1700). The module itself lives outside the framework repo.

• Add sync as an experimental fourth value on the tables page and the model reference, with the experimental_features: databaseSync: true / --experimental-features databaseSync opt-in.
• Fix the two "no experimental features" statements.
• Add a short offline-sync section (or page under client-side database) covering the opt-in, createSyncSession, and a link to the serverpod_offline_sync README on pub.dev, marked experimental.