Add RustDesk Client documentation

[Copilot]

Adds documentation for the RustDesk Client (software ID 13) introduced in MichaIng/DietPi#7992, covering installation, configuration, and setup for both public and self-hosted server scenarios.

Changes

• docs/assets/images/dietpi-software-remotedesktop-rustdesk-logo.webp: WebP conversion retained as an asset, but the RustDesk Client section uses the same SVG logo as the RustDesk Server section
• docs/software/remote_desktop.md:
  • Added RustDesk Client to the overview list
  • New ## RustDesk Client section with:
    • Intro referencing RustDesk Server relationship and X11 desktop requirement
    • ### Client management tabs: Client configuration, Update
    • ### Client installation and setup with two tabs:
      • Using the public RustDesk server: default setup via rs-ny.rustdesk.com:21116 (~/.config/rustdesk/RustDesk2.toml)
      • Using a self-hosted RustDesk Server: Settings > Network configuration (ID server + public key), plus an optional Persistent background service subsection explaining systemctl enable --now rustdesk — only recommended with a self-hosted server, as it keeps a persistent connection open for always-available remote access
• docs/software.md: Added RustDesk Client entry to the software catalogue list

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[StephanStS]

@MichaIng : The checks output
[WARN] lychee detected 154 redirects. You might want to consider replacing redirecting URLs with the resolved URLs. Run lychee in verbose mode (-v/--verbose) to see details about the redirections.
Should we look at the redirects and have them corrected (check before instead of blind replacement)?