Docs: refresh Linux installer setup and troubleshooting guidance

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Summary

Updates installer-focused documentation based on recently merged juice-oss changes (PRs #6, #7, #9, #11), while keeping updates scoped to existing docs pages.

Docs Updated

• docs/telemetry-program-setup.md
  • Added installer environment variable reference (INSTALL_JUICE_TOKEN, INSTALL_JUICE_POOL, INSTALL_JUICE_CONTROLLER, INSTALL_JUICE_VERSION)
  • Added custom controller install example
  • Added installer validation behavior for glibc and client/agent dependency checks
• docs/juice/user-guide/setup/systerm-reqs.md
  • Added explicit installer glibc constraint (>= 2.27)
  • Clarified dependency checks done by installer for Linux
  • Added libnvidia-encode as an agent-side runtime requirement supplied by NVIDIA drivers
  • Clarified behavior difference when INSTALL_JUICE_POOL is present vs omitted
• docs/juice/troubleshooting/troubleshooting.md
  • Added troubleshooting entries for:
    • installer success without service creation when INSTALL_JUICE_POOL is omitted
    • dependency validation failures (Please correct the above errors)
    • token check failures when using non-default controller without INSTALL_JUICE_CONTROLLER

Codepaths Covered

• juice-oss/installer/install-juice.sh
  • verify_libaries() dependency and glibc checks
  • setup_env() controller selection (INSTALL_JUICE_CONTROLLER)
  • create_users_and_directories() service install behavior and fatal-on-failure
  • install flow branch that skips service setup when INSTALL_JUICE_POOL is unset
  • service command change removing forced --log-level info

Knowledge Gaps Addressed

• Clarifies that installer success does not always mean an agent service was created
• Clarifies when missing agent libraries are warnings vs hard errors
• Documents custom controller usage for token/release API calls
• Makes glibc/dependency constraints explicit before install

Validation

• npm install && npm run build completed successfully in this branch.