Document Windows NPM setup and make sure it starts

[mx-psi]

What does this PR do?

• Add documentation about the need to use windows_npm_install when setting up NPM on Windows.
• Restart Datadog Agent service on Windows on system probe configuration changes.

Motivation

• Flag was undocumented and we did not mention this explicitly anywhere.
• The NPM driver did not start since the system probe class was applied after the service start.

Additional Notes

It's difficult to do this automatically since we would have to call the datadog::datadog_agent class on the datadog::system_probe one, so instead we are just documenting this.

Describe your test plan

Docs only change.

[buraizu]

Approving, but left a small suggestion for consistency with other parameter descriptions

[KSerrania]

I've left some comments about the documentation, but otherwise this looks great!