feat: add one-page cheatsheet in `getting started`

I spoke with @gakonst about merging the [Foundry Cheatsheet](https://github.com/dabit3/foundry-cheatsheet) into Foundry Book.

I put this repo together to have a more concise one-page overview of some of the things I was using the most, and many people ended up finding it helpful.

But it makes more sense not to have it separate from the rest of the learning material in Foundry Book, as to have less fragmentation.

I wanted to know what you all think of adding this to the Foundry book, and if so where you might think it should go?

Thanksk