[#70] add check for docs content in Indigo's example contracts test

Description

This keeps the existing testing for Indigo's examples and adds a check for each contract (Indigo-Lorentz couple) that verifies the content of documentation using their MarkDown representation.

NOTE: the new check does not compare ContractDocs directly because they do not have an Eq instance and it seems that in other documentation-related datatypes the Eq instance does not consider the textual content of documentation, but only its structure. Moreover the MarkDown representation is consistent, so it does not seem necessary to add a specific content-checking function.

Related issue(s)

Resolves #70 (closed)

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