Spike in inverter output waveform due to Cgd coupling
In order to change the voltage across a capacitor, you need current:

\$i_C=C\frac{dv_C}{dt}\$

The capacitor \$C_{gd}\$ connects input to output. So if the input were to change instantaneously from 0V to VDD, the voltage over \$C_{gd}\$ would have to change from VDD (\$v_{out}=VDD, v_{in}=0V\$) to -VDD (\$v_{out}=0V, v_{in}=VDD\$) instantaneously. This could only happen if an infinite amount of current flows through the capacitor!

As the input suddenly rose from 0V to VDD, the NMOS transistor switched on, and the PMOS transistor switched off. As you may know, the NMOS transistor will be in saturation, and will sink a pretty constant current to ground. This current is far from infinite! And so the capacitor can only slowly decrease its voltage. Instantaneously, it should retain all its charges as it hasn't had time to lose them through the NMOS. So at the exact instance where the input switches, the output will have to follow by the same amount of voltage.

Instantaneously, output will jump up from \$VDD\$ to \$2\cdot VDD\$... if no other capacitances are present.

If there are capacitors connected to the output, then they will not like the output to be changed instantaneously because it would mean they need an infinite current as well. So \$C_{gd}\$ will have to "fight" over the output voltage with all other capacitances connected to the output (actually, the charges stored on the capacitances are redistributed, this redistribution of charges will result in an infinite instantaneous current).

The output voltage they will agree on will depend on the capacitance of those capacitors. In the end, this causes the output not to jump over the full \$VDD\$, but some smaller voltage.

You will see that if you increase the capacitance at the output, that the peaking will get smaller and smaller. Unfortunately, the slew rate at the output will also get smaller and smaller...

The following figure summarizes everything into one image. \$C_L\$ is the total load capacitance, which is the sum of all capacitances connected to a constant voltage.