Add docs for pre-existing Active Directory user for unprivileged agent

[kilfoyle]

Mark as technical preview until AD support can be fully tested.

Use case:

For example, if I have a service account in active dieectory that works on the system already, can I use that or is elastic creating an account and group locally?

The use case is integrations that need network access from specific service accounts in active directory to be the service context for filebeat (api, file shares, etc).

Today system is the only option but this gives me hope for 1 non system which is great, but having custom users would be excellent.

In the past you could simply install filebeat or winglogbeat with the service account you needed as a service and it would use that user context when it runs.

Details are here and here.