Improve documentation of the crate

[DDtKey]

We need to enrich the crate with documentation

Depends on #1

Tasks

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1. Contribution guide - @ubiratansoares is assigned
2. Improve crate-level documentation (docs.rs)
3. Document usage of each existing module
4. Align readme with documentation
5. New issue template
6. Activate #![deny(missing_docs)] at crate level to avoid missing documentation

Anything I can do to help?