docs: ICA Overview (#626)

* docs: ica overview

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docs/app_modules/interchain-accounts/overview.md

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+# Overview
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+Learn about what the Interchain Accounts module is, and how to build custom modules that utilize Interchain Accounts functionality {synopsis}
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+# What is the Interchain Accounts module?
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+Interchain Accounts is the Cosmos SDK implementation of the ICS-27 protocol, which enables cross-chain account management built upon IBC. Chains using the Interchain Accounts module can programmatically create accounts on other chains and control these accounts via IBC transactions. 
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+Interchain Accounts exposes a simple-to-use API which means IBC application developers do not require an in-depth knowledge of the underlying low-level details of IBC or the ICS-27 protocol. 
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+Developers looking to build upon Interchain Accounts must write custom logic in their own IBC application module, called authentication modules.
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+- How is an interchain account different than a regular account?
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+Regular accounts use a private key to sign transactions on-chain. Interchain Accounts are instead controlled programmatically by separate chains via IBC transactions. Interchain Accounts are implemented as sub-accounts of the interchain accounts module account. 
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+# Concepts 
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+*Host Chain*: The chain where the interchain account is registered. The host chain listens for IBC packets from a controller chain which should contain instructions (e.g. cosmos SDK messages) for which the interchain account will execute.
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+*Controller Chain*: The chain registering and controlling an account on a host chain. The controller chain sends IBC packets to the host chain to control the account. A controller chain must have at least one interchain accounts authentication module in order to act as a controller chain. 
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+*Authentication Module*: A custom IBC application module on the controller chain that uses the Interchain Accounts module API to build custom logic for the creation & management of interchain accounts. For a controller chain to utilize the interchain accounts module functionality, an authentication module is required.
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+*Interchain Account*: An account on a host chain. An interchain account has all the capabilities of a normal account. However, rather than signing transactions with a private key, a controller chain's authentication module will send IBC packets to the host chain which signals what transactions the interchain account should execute.
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