Mounting a NFS share should be an easy task, however you might fail to mount the NFS share.
The ESXi vmkernel.log might show this error:
WARNING: NFS41: NFS41ExidNFSProcess:2054: Server doesn't support the NFS 4.1 protocol
In fact the warning message is correct. As of DSM version 7.0.1 only NFS3 and NFS4 are supported but NOT NFS version 4.1

The problem was discussed on Reddit and the community came up with a solution.

On the Synology NAS edit /etc/nfs/syno_nfs_conf and set nfs_minor_ver_enable=1.
After restarting the NFS service using systemctl restart nfs-server you should be able to connect to the NFS share from your ESXi server.