If I understand well your use case, you want to use Dialogflow to get what the user is asking for (example: tool, subject..ect), then use that to fetch a related Document from your Google site?
What do you mean by “I don't want to improve knowledge database with our documentation ” ?
I suppose when you should start by training an agent that identify the key words that your documents are about [1][2] (this should be your entities ) then you connect the agent to your service (example Webhook) to get the link to your document.
The difficult question is once you know what the user want, how do you search for the related documents? Can you provide more detail on how your document are sorted ? do you have a database that holds information on each document?
You may want to check this third party example [2] even though it's not exactly your use case, but it shows how to use a Webhook.
[1]: https://medium.com/swlh/how-to-build-a-chatbot-with-dialog-flow-chapter-1-introduction-ab880c3428b5
[2]: https://cloud.google.com/dialogflow/docs/quick/build-agent