Add documentation about **Coin Shuffle** flow and our implementation

[Velnbur]

Situation

Currently, the description and explanation of Coin Shuffle flow is distributed across comments and documentation of code from all Coin Shuffle repos, which makes it hard for new maintainers to start to collaborate and help with new tasks

Objective

Write a simple, but fully covered documentation about Coin Shuffle flow

Acceptance

It could be a markdown file or files in .github repo with separate directory like docs which will contain a tutorial like documentation with examples, charts and images