[#180] Add contract documentation tutorial to Indigo website

Description

Problem: The first problem is that documenting entrypoints requires defining instances of a big typeclass, which is scary-looking for beginners, especially if they are not Haskell-friendly.

The second problem is that, while there are ways to print and create contracts from ghci explained in the tutorial, there is no explanation of docs in it or method to interact with them.

Solution: Add documentation tutorial including the interacting docs via the REPL.

Related issue(s)

Resolves #180 (closed)

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