In Synology Diskstation you first have to enable the "WebStation" service. "WebStation" is the name for the webserver in Synology. As soon as you have enabled the WebStation, you can put the files for HTTP validation in place.
You can find the instructions for configuring Synology WebStation here:
http://blog.e-nnov.fr/en/synology-dsm-en/webstation/
For further details regarding SSL installation please check the Synology knowledge base:
https://www.synology.com/en-us/knowledgebase/