Update Linux installer and Docker agent docs

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Summary

• Updated Linux installation docs with verified installer modes for client-only installs and agent service installs.
• Expanded Docker agent docs with compose, docker run, runtime parameter, update, and custom build guidance.
• Refreshed Linux system requirements and troubleshooting for installer dependency checks and Docker UDP gateway setup.

Codepaths Covered

• installer/install-juice.sh: INSTALL_JUICE_TOKEN, optional INSTALL_JUICE_POOL, custom controller/version/user/restart options, pool names with spaces, glibc and dependency checks, systemd/OpenRC service behavior.
• docker/agent/README.md and docker/agent/root/home/juice/start-agent.sh: JUICE_TOKEN, JUICE_POOL, JUICE_HOST_IP, JUICE_HOST_PORT, JUICE_ADDITIONAL_ARGS, compose and CLI flows.
• docker/agent/Dockerfile: Ubuntu 24.04 base, CUDA 13 runtime package installation, Video Codec SDK build input.

Knowledge Gaps Addressed

• Clarifies that omitting INSTALL_JUICE_POOL installs only the CLI and does not create an agent service.
• Documents agent-only Linux dependency failures, including libnvidia-encode.so.
• Replaces stale Docker host-network-only guidance with reachable UDP gateway guidance.

Verification

• npm ci && npm run build passed. The build still reports existing site-wide edit-link warnings.