Update Gas Overview

[MBerguer]

In #55, there was one bullet point that we decided to handle in a separate PR because it required some coordination.

Update Gas Overview section with changes introduced in: matter-labs/foundry-zksync#773

Also, since this is an overview, let's see if it is possible to separate this into three subpages, as it would be good to talk about how this works in general, similar to how it works in the ZKsync Specifics section.