Document LambdaBuffers packages (Prelude and Plutus)

[bladyjoker]

When implementing runtimes for LB packages (Prelude, Plutus), we need to have a 'language neutral' specification of:

1. Which opaques types are listed and what do they map to in different languages.
2. How opaque types support a type class
   • Json encodings
   • PlutusData encodings
   • Equality
3. How transparent types support a type class via derivation
   • Given a sum/record/prod type how does a Json/PlutusData encoding look like? How is equality performed?

Let's add necessary documentation for this.

I sense that documentation should be associated with an LB package. So it's basically a documentation for the LB package.

• docs/lb-prelude.md
• docs/lb-plutus.md