Lite docs: sync and backup

[fylorn]

Waits for the Lite release that ships sync and backup; do not merge before.

Summary

• New page: Sync and backup (/docs/lite/sync-and-backup, English and Chinese), in Get started after Connecting to a remote core:
  • configuration packs (.twconfig) and backups: the last 10 versions on the device, the latest and the previous one in the sync location;
  • exporting and importing: keys included by default, optional passphrase encryption, per-item Overwrite / Import renamed / Skip for conflicts, plugins imported disabled, ChatGPT upstreams signed in again;
  • turning on sync with a folder, WebDAV or S3-compatible storage, and how to set up iCloud Drive, Dropbox, OneDrive, Google Drive, Jianguoyun (app password), Nextcloud, Synology, Cloudflare R2, Alibaba Cloud OSS, Tencent Cloud COS and MinIO, with generic endpoint formats;
  • encryption and what a lost passphrase means, what is synced (including upstreams not synced), merge units and conflicts, sync states, stopping sync;
  • restoring a whole version or only some items, safe mode, connected clients, remote cores, the twcore commands, and the table of what is never exported or synced.
• Import links: a "Generated by Lite" section for the upstream Share dialog: keys included by default with the notice, what the link carries, and the configuration pack when the link would exceed 8192 characters.
• Features: a Sync and backup section, the Share and Export selected entries under Upstreams, and the new settings section.
• Overview, Connecting to a remote core, Architecture, FAQ: one-line updates where sync and backup changes what they say (the settings groups, history and rollback becoming backups, where data is kept and what the app connects to).
• Sidebar entry and summaries in src/content/docs/_meta.ts.

Testing

• pnpm build passes locally; every in-page and cross-page docs link with a fragment resolves.

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