Add whitepaper to /doc

whitepaper is incomplete, is a rough draft.

Goal of this whitepaper is to explain in detail the entire inner workings of Sia, such that someone could design an equivalent system from just the whitepaper.

The whitepaper is not a protocol specification, and is not meant to define the Sia protocol at the byte level. It is only meant to cover academic concepts and proofs.

There was also an update to the .gitignore to prevent latex output from being added to the repo in the doc/ directory.