Add documentation

[franziskuskiefer]

Add documentation how to get started with this repository.

Documentation

• readme
• blog: https://github.com/hacspec/blog | https://hacspec.org/blog/
• website: https://github.com/hacspec/hacspec.github.io | https://hacspec.org/
• book: https://github.com/hacspec/book | https://hacspec.org/book/
• rust docs: on all crates, especially on cargo-hax
• ocaml docs (see issue OCaml: add docstrings on modules #436)
• examples & demos: https://github.com/hacspec/hax/tree/main/examples

Tasks

• installation instructions
• cli instructions
• update/replace book
  • User
    • Examples
    • Tutorials
    • Proofs
      • F*
      • Coq
      • libcore
  • Contributing
    • Structure
    • Cargo command
    • Frontend
    • Engine
    • Backends
    • Utilities
    • Library & Macros
    • libcore
• Architecture & Design technical documentation
• Architecture & Design blog post
• Update website
• Update readme