Document Linux agent installer options

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Summary

• Added Linux Agent service installer usage to the Installation guide, including M2M token setup, quoted pool-name examples, additional service args, installer options, and uninstall guidance.
• Updated System Requirements with source-verified Linux installer checks for systemd/OpenRC, curl/wget, glibc 2.27+, and Agent-only libnvidia-encode behavior.
• Added troubleshooting guidance for installer pool-name quoting, missing dependency failures, and default service log locations.

Codepaths Covered

• Juice-Labs/juice-oss PR Verify for google crawler' #12: installer/install-juice.sh quoting of INSTALL_JUICE_POOL for pool names with spaces.
• Juice-Labs/juice-oss PR Update Manually Managing Sessions #11: installer/install-juice.sh generated service command no longer forcing --log-level info, plus clearer dependency check output.
• Current juice-oss installer README and installer/install-juice.sh service install flow.

Validation

• Ran npm ci && npm run build successfully.
• Browser-verified the rendered Installation, System Requirements, and Troubleshooting pages against the locally served Docusaurus build.

Note: direct Juice-Labs/juice repository access was unavailable to this Cloud Agent, so behavior was verified against the accessible Juice installer source in Juice-Labs/juice-oss, which is linked from the docs.