[#277] Add good example contracts to the Indigo website

Description

Problem: People want to see examples of Indigo smart contracts, but our website does not have anything beside toy examples. We have some examples that demonstrate how Indigo can be used to write non-toy contracts, we should make it easy to find them. If you consider using Indigo for your non-toy project, you want to see examples of successful non-toy smart contracts.

Solutions: Add examples of Indigo smart contracts in the Indigo doc such as:

• AbstractLedger and ManagedLedger
• Globacap

Related issue(s)

Resolves #277 (closed)

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