ONYX-11477: Update Ruby conventions page

[tzheleznyak]

Updated ruby conventions for conjur with things i learned during the process of development of the jwt authenticator in Conjur. Learned for pr comments , code climate issues and conversations with:

@orenbm @abrahamko @nessiLahav @Avi-Eyal @sashaCher @benoaviram @jvanderhoof @jonahx

Changelog

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Test coverage

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Documentation

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Behavior

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Security

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