Rewrite offline-database article with modern sync section and improvement guide (#8939)

* Rewrite offline-database article with modern sync section and improvement guide

Full rewrite of docs-src/docs/articles/offline-database.md:
- New structure: definition of an offline database, benefits, RxDB intro,
  setup example, Sync Engine, encryption, improvement steps, FAQ.
- Sync section now describes the modern RxDB Sync Engine: git-like
  document protocol, pull/push handlers plus pull stream, checkpoint
  iteration and event observation modes, client-side conflict
  resolution, batching, leader election, and the current list of
  replication plugins (Supabase, Appwrite, MongoDB, NATS, Firestore,
  CouchDB, GraphQL, HTTP, WebSocket, WebRTC, RxServer).
- New section with eight suggestions on how to improve an offline
  database setup: storage choice, partial sync, cleanup, key
  compression, worker storages, schema migrations, conflict handling,
  and knowing the tradeoffs.
- Rewritten FAQ targeting real search queries with verdict-first
  answers.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FunfQhtwvQumvwSbU2k8gF

* Shorten code block line to pass docs line-length check

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FunfQhtwvQumvwSbU2k8gF

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Co-authored-by: Claude <noreply@anthropic.com>

Author: pubkey