Documentation: Document tee command.

[Zepp-Hanzj]

Summary

• Document the tee system command page that was previously blank.
• Add configuration, usage, options, arguments, examples, and notes backed by apps/system/tee/tee.c and apps/system/tee/Kconfig.

Impact

• Documentation only.
• No runtime behavior changes.

Testing

• Verified source behavior in apps/system/tee/tee.c and configuration in apps/system/tee/Kconfig.
• Ran focused RST parse:

  python3 - <<'PY'
  from pathlib import Path
  from docutils.core import publish_doctree
  path = Path('Documentation/applications/system/tee/index.rst')
  publish_doctree(path.read_text(), source_path=str(path),
                  settings_overrides={'halt_level': 2, 'report_level': 2})
  print('docutils OK')
  PY

• Ran full documentation dummy build:

  cd Documentation
  PYTHONPATH=_extensions python3 -m sphinx -W -b dummy . /tmp/nuttx-docs-build-tee

PR verification Self-Check

• The change is limited to documentation.
• The documented behavior is source-backed.
• The documentation build completed without warnings.
• No external dependency or board-specific setup is required.

Refs #11081