Systemd is able to handle various different service types specifically one of the following
simple - A long-running process that does not background its self and stays attached to the shell.
forking - A typical daemon that forks itself detaching it from the process that ran it, effectively backgrounding itself.
oneshot - A short-lived process that is expected to exit.
dbus - Like simple, but notification of processes startup finishing is sent over dbus.
notify - Like simple, but notification of processes startup finishing is sent over inotify.
idle - Like simple, but the binary is started after the job has been dispatched.
In your case you have picked
Type=forking which means systemd is waiting for the process to fork itself and for the parent process to end, which it takes as an indication that the process has started successfully. However, your process is not doing this - it remains in the foreground and so
systemctl start will hang indefinitely or until the processes crashes.
Instead, you want
Type=simple, which is the default so you can remove the line entirely to get the same effect. In this mode systemd does not wait for the processes to finish starting up (as it has no way of know when this has happened) and so continues executing and dependent services straight away. In your case there are none so this does not matter.
A small note on security:
You are running the service as root, this is discouraged as it is less secure than running it as an unprivileged user. The reason for this is that if there is a vulnerability in jekyll that somehow allows execution of commands (possibly via the code it is parsing) then the attacker needs to do nothing else to completely own your system. If, on the other hand, it is run as a non-privileged user, the attacker is only able to do as much damage as that user and must now attempt to gain root privileges to completely own your system. It simply adds an extra layer attackers must go though.
You can simply run it as the same user that is running your web server, but this leaves you open to another potential attack. If there is a vulnerability in your web server that allows the user to manipulate files on your system they can modify the generated html files, or worst the source files and cause your server to serve anything they want. However, if the generated files and source files are only readable by the webserver and writable be another non-privileged user they will not be able to, as easily, modify them by attacking the web server.
However, if you are simply serving static files from this server and keep the server up to date these attacks are very very unlikely - but still possible. It is your responsibility to weigh the risks vs the overhead of setting it up based on how critical your system is but both of these tips are very simple to set up and next to no maintenance overhead.