Hi @Heiko-san,
At the moment, the documentation you see on the Terraform website is maintained manually in Markdown format. There is no automatic tool for generating that documentation from the schema, because the documentation usually includes additional information that the schema cannot represent.
From Terraform v0.12.0 onwards there is a tool built in to Terraform for retrieving provider schemas in JSON format: terraform providers schema -json. As far as I know, nobody has yet written the second tool that would take that result and render it to HTML or to Markdown.