Intro text

I think intro text would help make this even better as a standalone learning experience, in 2 areas:

### 1. What does this thing do?
It could take a few forms. In order of my personal preference:

- small section that's visible by default, and doesn't impede usage of visualizer, but can be dismissed via click or scroll
- in the header, always visible
- triggered by a help or about link
- splash screen upon first load

Here's a first pass of the text, interested in comments!

"DAGs (directed acyclic graphics) are the core data structure of IPFS. They enable efficient data verification and management. Upload a file, then click menu options to explore the different ways that file can be split up into chunks."

### 2. What do these things mean?
The menu options might also benefit from some description. Example below:

Start by opening the DAG visualizer and reviewing the header, initial view, and menu options described in the issue. Decide how the introductory explanation and menu descriptions fit without impeding use. Done means the visualizer explains what it does and what its options mean through an accessible intro or help interaction.

Written by the indexing model from the issue text.