Move sidekick-docs to docs repository

[etabakov]

Now that we have greatly simplified the build procedure and infrastructure, it would be easy to migrate the markdowns of sidekick's documentation to the main docs repository.

Benefits:

• a little bit easier local setup
• a little bit faster build
• all resources (template, styles, scripts) will be reused and we will no longer have to make changes on two places
• on PR users will receive results from the build and errors in the markdown will be caught earlier