Replace redundant table of contents in Concepts and Tutorial documentation #232

https://docknetwork.github.io/sdk/tutorials/concepts.html and https://docknetwork.github.io/sdk/tutorials/tutorials.html contain tables of contents, but that's not needed because mdbook already generates a TOC on the lefthand side of the page.

You'll notice the above ToC is out of date. That's because it's not automatically generated.

We should remove the extra ToCs and write intros to the Concepts and Tutorial sections like in the rustinomicon.