In-depth review of general documentation

[adpaco]

Including the following:

• README.md file
• Source code documentation.
• Kani docs

[adpaco]

For source code documentation, we would need to make clear what the scope according to the current layout:

• Code under src (kani-compiler, cargo-kani and cargo-metadata).
• cprover_bindings
• Kani docs (i.e., files under docs/src)
• Kani libraries (i.e., files under kani/library: The kani library, kani-macros and std overrides)
• Scripts
• Tests
• Tools (compiletest and bookrunner)

Note: We agreed on a team discussion that source code documentation is out of scope for this issue.