Add motivation docs

[Melevir]

Add article on comparing cognitive complexity to cyclomatic, show code samples, explain reasoning, show use cases.

[pkolbus]

I've found G. Ann Campbell's white paper to be quite readable and seems to cover these aspects. Is there something I'm missing?

[Melevir]

Original paper is very informative if you have read original paper :)

I think it would be nice for docs to have simple explanation of what is cognitive complexity, with reasoning, samples, etc. We can't make anyone read Campbell's paper.

[pkolbus]

Thanks for clarifying- a simple introduction does make sense to add.

[Melevir]

This one is still valid.