Document Linux installer and agent build updates

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Summary

• Updated Linux installation docs with the one-step agent installer flow, including INSTALL_JUICE_TOKEN, optional INSTALL_JUICE_POOL, custom controller support, pool names with spaces, and forwarding service options after sh -s -.
• Updated Linux system requirements with installer dependency behavior for glibc 2.27+, client libraries, and agent-only libraries such as libnvidia-encode.
• Updated Docker agent docs with custom build artifacts required by the source Dockerfile/README and added --gpus all to the run example.

Codepaths covered

• Verified against recent accessible Juice source changes in Juice-Labs/juice-oss: installer/install-juice.sh and docker/agent/{Dockerfile,README.md}.
• Direct Juice-Labs/juice access returned repository not found in this environment, so source verification used the public Juice OSS repository that backs the linked installer/Docker docs.

Knowledge gaps addressed

• Clarifies that the Linux installer can do a client-only binary install when INSTALL_JUICE_POOL is omitted.
• Explains how to quote pool names with spaces and how to pass agent service flags to the installer.
• Documents why missing agent libraries may be warnings or fatal errors depending on whether the service is being installed.
• Documents required local artifacts before building a custom agent image.

Validation

• npm ci
• npm run build (passes; existing site-wide edit-link warnings remain)