docs: init mkdocs

[tamasmagyar]

Initial commit for Mkdocs documentation migration.

Example deployed version can be reviewed here: https://tamasmagyar.github.io/cypress-image-diff/

Will extend contribution documentation based on mkdocs get started in the next PR

• https://www.mkdocs.org/user-guide/deploying-your-docs/#github-pages

Uncertainty: Not sure if I can run mkdocs gh-deploy command for the uktrade/cypress-image-diff repository

[tamasmagyar]

we are going to use gitbook instead of mkdocs