Link 'Build on Koinos' to 'https://docs.koinos.io/developers/'

[MiXiBo]

Resolves

Brief description

Checklist

• [x ] I have built this pull request locally
• [x ] I have ran the unit tests locally
• [ x] I have manually tested this pull request
• [ x] I have reviewed my pull request
• [ x] I have added any relevant tests

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[sgerbino]

Closing due larger site overhaul