docs: clarify Linux installer controller/pool behavior and prerequisites

[cursor]

Summary

Updated existing docs to reflect recently merged Linux installer behavior in Juice-Labs/juice-oss.

Docs updated

• docs/juice/user-guide/setup/installation.md
  • Added a Linux one-step installer section (curl https://get.juicelabs.co | ... sh -)
  • Documented installer env vars used by current script:
    • INSTALL_JUICE_TOKEN (required)
    • INSTALL_JUICE_POOL (optional)
    • INSTALL_JUICE_CONTROLLER (optional)
  • Added concrete usage examples for:
    • client-only install (no pool)
    • service install with pool
    • custom controller
• docs/juice/user-guide/setup/systerm-reqs.md
  • Added Linux one-step installer prerequisites:
    • glibc >= 2.27
    • systemd or openrc
    • curl or wget
• docs/juice/troubleshooting/troubleshooting.md
  • Added Linux installer troubleshooting checks for token/controller validation, glibc floor, and required libs.
  • Clarified expected behavior when INSTALL_JUICE_POOL is omitted (binaries only; no service created/started).

Source codepaths covered

All doc claims were verified against Juice-Labs/juice-oss installer source and recent merged PRs:

• installer/install-juice.sh
  • setup_env() (INSTALL_JUICE_CONTROLLER default/override)
  • verify_token() (controller-aware token check)
  • verify_libaries() (glibc minimum and dependency checks)
  • main install flow conditional on INSTALL_JUICE_POOL for service creation/start
• Recently merged PRs reviewed:
  • Update Manually Managing Sessions #11 Remove log level from service command
  • Update cli-login.md. Correct m2m documentation, make it consistent #7 Add support for custom controller to install script
  • Update index.md #6 Decrease min glibc version and make juice pool optional

Validation

• Ran docs build locally with npm run build.
• Build succeeds; there are pre-existing broken-link warnings unrelated to this change.