docs: add DEP introduction

[audtlr24]

Add DEP introduction to 0-about-panacea/1-data-exchange-protocol.

• what is dep?
• dep and problems it solves
• how dep works

There's a lot of overlap with what was in the old docs, but I haven't deleted it yet. I will delete or edit after get comments.

Then, I edit the detailed docs about our new specs.