Add limitations for Azure

As more and more customers are using Azure, we should have a page listing the limitations.

For now, we've identified a first one about uploads :

• Azure Application Gateway (Apache-like software) has the "Maximum file upload size, Standard" set to 2GB

It's described at https://github.com/MicrosoftDocs/azure-docs/blob/master/includes/application-gateway-limits.md

and confirmed by Microsoft with

Maximum file upload size: 
Standard, 2 GB
V1 Medium WAF gateways, 100 MB
V1 Large WAF gateways, 500 MB
V2 WAF, 750 MB

So, until Nuxeo provide multi-part upload with other storage than S3 (where we can leverage multi-part upload capabilities from AWS with S3 direct upload), it won't be possible to upload to Azure files larger than 2GB