docs: update Linux installer and agent Docker troubleshooting guidance

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Summary

This docs-only PR updates existing documentation pages to match recently merged installer and agent-image behavior from Juice-Labs/juice-oss.

Docs added/updated

• Updated docs/juice/user-guide/setup/installation.md
  • Added a Linux Agent Installer section covering required/optional installer env vars and service install behavior.
  • Added concrete installer examples for service setup and runtime argument forwarding.
• Updated docs/juice/troubleshooting/troubleshooting.md
  • Added Linux installer troubleshooting entries for dependency checks, token/controller validation failures, and binary-only installs when no pool is supplied.
• Updated docs/juice/user-guide/docker/docker-agent.md
  • Added custom image build prerequisites (juice-gpu-linux.tar.gz and Video_Codec_SDK_<version>.zip) and VIDEOSDK_VERSION build-arg usage.
• Updated docs/juice/user-guide/setup/systerm-reqs.md
  • Added explicit glibc >= 2.27 requirement for Linux installer-based setups.

Codepaths covered

These docs map to merged changes in Juice-Labs/juice-oss:

• installer/install-juice.sh
  • Optional INSTALL_JUICE_POOL service setup behavior
  • INSTALL_JUICE_CONTROLLER support for custom controller host
  • Dependency validation behavior and glibc minimum
  • Agent service invocation argument handling (agent run runtime args)
  • Service install fatal behavior when agent service install fails
• docker/agent/Dockerfile and docker/agent/README.md
  • Custom build artifact expectations and Video Codec SDK versioning details

Key knowledge gaps addressed

• Clarified why installer can complete without creating a running service.
• Clarified controller/token validation failure causes and recovery checks.
• Clarified Linux dependency constraints that commonly block installs.
• Clarified custom Docker agent build inputs that are required but easy to miss.

Validation

• Ran npm run build successfully in juice-docs.
• Build reports pre-existing global broken edit links unrelated to this change set.