[Fleet] Add variable to the preconfiguration example

[nchaulet]

Description

Resolve elastic/kibana#159444

Add variable to the preconfiguration example

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[mergify]

This pull request does not have a backport label. Could you fix it @nchaulet? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-/d./d is the label to automatically backport to the /d./d branch. /d is the digit
  NOTE: backport-skip has been added to this pull request.

[nchaulet]

@kilfoyle will it make sense to add something to tell the user they can find all the available input and variable in the integration UI?

[kilfoyle]

@nchaulet I think that makes sense. Do you think it might be better to refer people to the Integrations documentation rather than the UI? Usually the docs are better as a reference, but I don't see that period setting described on the System integration page.

Depending on which is better (refer people to the docs or to the UI), here are a couple of suggestions for the text.

Refer users to the docs:

The following example creates an agent policy for general use, and customizes the `period` setting for the `system.core` data stream. All available inputs and variables are described in the link:https://docs.elastic.co/en/integrations[Integrations] documentation.

Refer users to the UI:

The following example creates an agent policy for general use, and customizes the `period` setting for the `system.core` data stream. You can find all available inputs and variables in the **Integrations** app in {kib}.

[nchaulet]

Do you think it might be better to refer people to the Integrations documentation rather than the UI? Usually the docs are better as a reference, but I don't see that period setting described on the System integration page.

Unfortunately I do not think we have the information in the Integrations documentation

[kilfoyle]

LGTM! 🚢
Thanks @nchaulet!