Contextualize IPFS workshop

[`tutorials/ipfs`](https://github.com/CodeForPhilly/decentralized-data/blob/gh-pages/tutorials/ipfs/index.md) doesn't provide any context: what is IPFS? Why are we learning about it? The workshop page should provide some framing. (This issue is inspired by Ib's question during the workshop.)

Start with tutorials/ipfs/index.md and review the existing workshop flow. Add introductory framing that explains what IPFS is and why the workshop uses it, then read the rendered page to confirm the context is clear before the exercises begin.

Rédigé par le modèle d'indexation à partir du texte de l'issue.