Update documentation about Presigned-URLs of Object Storage

[lmfjd]

As an OVHcloud customer using the object storage I want its documentation to be updated with concrete mentioning of pre-signed URL features like the POST, so that I don't have to search half a day just to find out that it does not work.

I found out that pre-signed URLs for POST is not available by stumbling upon this issue: https://github.com/ovh/public-cloud-roadmap/issues/317 In the official documentation of the S3 compliancy there is no mention of the term "pre-signed-URL" as amazon refers to it. It is only mentioned "Web Content" and "Static Web Site".