update Req HttpClient example

[woylie]

This updates the Req example in the ExAws.Request.HttpClient documentation.

• Set decode_body: false to prevent Req from decoding JSON responses on its own (behaviour expects binary body).
• Rename status to status_code in response to conform to behaviour. I know ex_aws was updated to map the response on its own in the main branch, but the behaviour still prescribes a status_code field.
• Wrapped error into reason map as prescribed by behaviour.
• Added missing @impl.
• Added moduledoc to example for easier copy-and-paste into projects that enforce module docs with linter rules.
• Updated headers to avoid duplicate links in the sidebar.

[bernardd]

Thanks @woylie! Apologies for the delay in getting to this.