Document new Windows service support

[binarylogic]

Document the changes in https://github.com/timberio/vector/pull/2896.

[pront]

:claude:

 Evidence of Active Support:

  1. Code is maintained ✅
    - Files: src/service.rs and src/vector_windows.rs
    - Last updated: August 2025 (Rust 1.89 upgrade)
    - No deprecation notices or removal plans
    - Actively included in recent Rust edition upgrade (2024 edition)
  2. Dependencies are current ✅
    - windows-service = "0.8.0" in Cargo.toml (line 438)
    - This is a recent version of the windows-service crate
  3. Conditionally compiled for Windows ✅
    - Uses #[cfg(windows)] to ensure it only builds on Windows
    - That's why it doesn't show in the help output on macOS/Linux
  4. Full functionality available ✅
    - vector service install - Register as Windows service
    - vector service uninstall - Remove the service
    - vector service start - Start the service
    - vector service stop - Stop the service
    - vector service restart - Restart the service