AWS: Add documentation for x-amz-content-sha256 header

Thanks for creating this! It would be great to see documentation for the `x-amz-content-sha256` header in the AWS signer docs. It seems this field needs to be set manually for an empty body (and perhaps in other cases) and this is not evident from the main example in the README or elsewhere. My original assumption was that this header would be set by the signer, and it took some time to figure out that I needed to set it myself.

Start with the AWS signer documentation and the main README example, focusing on the x-amz-content-sha256 header and the empty-body case described in the issue. Update the documentation to explain when the header must be set manually, then verify that the example and surrounding guidance make this behavior clear.

Written by the indexing model from the issue text.