Add API Publisher Documentation

I am proposing the structure for API documentation:

• In the content directory, have a directory for publish-apis and subscribe-to-apis
• In each of those directories, have a one directory for each category: tutorials, how-to guides, explanations, and technical reference
• In each of those directories, have one directory for each "thing" - e.g. one directory per tutorial in the tutorials directory
• In each of those directories, have one markdown (index.md) containing the content and an images directory containing the images.

This merge requests tries to reflect this proposed structured.