This feature is actually available. When you assign CMS user (content manager) to one of your websites, a designated dashboard providing access to this site only is automatically created. You can learn more about the CMS dashboard in this link: http://knowledgebase.webydo.com/All-articles-/63879778/Add-Content-Manager-CMS-User-to-Site.htm.
This is a great feature, and we are considering to add it to our roadmap. Hopefully, with enough votes from our community it will get developed soon.