refactor: rename project to shroud

Rename project from outline-distro to shroud for better branding.

- Update module path in go.mod from outline-distro to shroud.
- Rename binary, service file, and all installation paths.
- Update all import paths across internal packages.
- Add comprehensive README.md with usage and architecture docs.
- Update install script and systemd unit file for new naming.
- Update config example with new cert cache path.

Initial commit: Shadowsocks + AmneziaWG VPN server

Single-binary Go replacement for the Outline VPN server stack.
Supports Shadowsocks (via outline-ss-server library) and AmneziaWG
with HTTP/3 mux for DPI resistance (MuxBind shares UDP port between
AWG and a real QUIC/HTTP/3 server with Let's Encrypt).

Includes systemd unit, install script for Ubuntu, and REST API
compatible with the Outline Manager.

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