Define Content Levels (L100-L400)

[simonua]

The API Management Hands-on Labs originally targeted Level 200. As more features have been released, and as we have observed much increased interest by adopters to provide short, digestible labs and examples, it makes sense to a) define what levels are, and b) which levels pertain to what content.

I am suggesting adding a Table of Contents which associates linked lab content with an appropriate level. Furthermore, I suggest adding clear definitions for each level, so that lab users may have it easier to understand their API Management learning journey.

Additionally, I propose adding a top-aligned, standard entry in each appropriate markdown file to communicate what level the content pertains to. I see this as being added underneath each page's header.