[DOCUMENTATION] Add installation and prerequisite guides for supported package managers #5516

Problem

UniGetUI lists supported package managers and may report missing dependencies, but there is no easily discoverable, centralized setup guide for each package manager, including required components, installation commands, verification steps, and common troubleshooting cases.

This creates a poor onboarding experience: users have to research and install dependencies individually even after UniGetUI identifies what is missing.

Concrete example: Cargo on Windows

While setting up Cargo support in UniGetUI on Windows:

1. Installing Rust/Cargo did not provide the additional cargo-update executable required for UniGetUI's update integration.
2. Attempting cargo install cargo-binstall --locked failed because the MSVC target needs Microsoft's link.exe, which was not installed.
3. After installing a prebuilt cargo-binstall, running cargo binstall cargo-update -y installed the two required binaries successfully (cargo-install-update.exe and cargo-install-update-config.exe).
4. Network/VPN-related DNS timeouts were a separate obstacle; retrying without VPN succeeded.

A short guide could have explained these prerequisites and offered a precompiled-binary path without requiring the Visual C++ Build Tools merely to set up package management.

Suggested improvement

• Provide a central Package manager setup / prerequisites page, linked prominently from the website and documentation.
• For each supported package manager, document installation on Windows, required runtimes/CLI extensions, any optional or mandatory dependencies, PATH requirements, and commands to verify the installation.
• Indicate which components UniGetUI can install/configure automatically and which the user must install separately.
• In UniGetUI's missing dependency warnings, link directly to the relevant guide, ideally including a safe copyable install command.
• Include short troubleshooting notes for common scenarios such as missing MSVC build tools, PATH detection after installation, and VPN/proxy/DNS failures.

Expected outcome

When UniGetUI reports a missing package manager component such as cargo-update, users should be able to click through to an official, package-manager-specific setup guide rather than search GitHub issues or third-party instructions.