Split README to simple landing page and documentation

[nettle]

Describe the task

We should split our long README.md into simple landing page and actually documentation.
Reason: README.md should be very simple (What it is, How to use, that's it).
For documentation I would suggest https://github.com/docsifyjs/docsify or similar

Scope

1. Move current README.md to doc/README.md
2. Create small main README.md with minimal information:
   • What it is
   • How to use
3. Optionally add simple documentation rendering engine
   • One of the options: docsify
4. Optionally add documentation publishing to CI
5. Plan next for documentation development

Definition of Done

• Simple README.md
• Documentation under doc/