Useful trick: This is a single NAND gate we're laying out.
When you're designing a system you will probably use many
of these gates connected to inverters and other gates.
In terms of making later alignment easy, setting a fixed
VDD-ground rail separation and input-output height is a
good idea. The NAND has two inputs, but at least one of them
can be set to the fixed height. See the next figure, with
all the important vertical dimensions that should be invariate from
cell to cell marked with rulers.