Refactor and document SampCert

[mjdemedeiros]

Refactor and document SampCert up to Mathlib4 standards.

Progress

• SLang.lean
• Foundations/*
• Samplers/*
• DifferentialPrivacy/*
• Extractor/*
• Util/*
• Extraction.lean

Extractor and CI work will be in a different PR

Tasks

• Document contribution guidelines (including protected names for extraction which violate the matlib naming scheme)
• remove explicit #lint docBlame (add linting to CI?)

[mjdemedeiros]

@jtristan Documentation is close to finished and should be merged before it goes too out of sync. Some functions (particularly in ZeroConcentrated/Postprocessing and Laplace/Properties) are not finished because I'm still digesting them.

I also wrote a CONTRIBUTING.md which briefly goes over the changes I made. If any of them are too controversial, I can change them back.