Add Download Client configuration docs

[Copilot]

This updates the Configuration docs to cover the new Download Client integration for qBittorrent. The page now documents when cleanup runs, which settings are available, the qBittorrent prerequisites for matching downloads, and the Sonarr-specific delete scope while preserving the required #download-client anchor used by the app.

• Configuration docs

  • Added a new top-level ## Download Client section to docs/Configuration.md immediately after ## Sonarr
  • Kept the heading text exact so Docusaurus continues to generate #download-client

• User-facing behavior

  • Documented that the Download Client settings page only appears after Radarr or Sonarr is configured
  • Explained that cleanup is matched via Radarr/Sonarr download history and only applies to media removed through those integrations
  • Added a prerequisite note that Maintainerr must target the same qBittorrent instance used by Radarr/Sonarr, with Web UI enabled on qBittorrent 4.1+
  • Clarified which collection actions trigger cleanup and which do not
  • Called out qBittorrent as the currently supported client

• Settings reference

  • Added a settings table for:
    • URL
    • Username / Password
    • Delete downloaded data
    • Fallback seeding ratio

• Operational constraints

  • Documented Sonarr’s whole-show-only cleanup behavior
  • Clarified that failures talking to the download client do not block Radarr/Sonarr deletions
  • Noted that Test Connection validates qBittorrent Web UI connectivity and credentials before saving

## Download Client

:::note
The separate `Settings -> Download Client` page only appears once Radarr or Sonarr is configured.
:::