Move docs to twilio.com/docs/authy/api; rewrite and standardize readme

[robinske]

Updates the README to be consistent with other Twilio supported helper libraries for Authy.

See: https://www.twilio.com/docs/authy/api/applications https://www.twilio.com/docs/authy/api/users https://www.twilio.com/docs/authy/api/one-time-passwords https://www.twilio.com/docs/authy/api/push-authentications

Also moves Verify V1 docs to a legacy readme and guides folks towards V2 of Verify

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