Update the Accounts and Permissions section

**What is the relationship between keys, eosio accounts, and smart contracts?**

EOSIO accounts identify unique individuals or entities within an EOSIO blockchain. Keys in EOSIO are binary strings represented in Base58 used for signing and verification of transactions, blocks, and other messages. Keys are created within a digital wallet associated with an account. Since account ownership is defined solely by the account name, the keys associated with an account can be updated without compromising security. A novel permission scheme involving accounts, permissions, and authority tables determine what accounts can do and how the actions that make a transaction are authorized. To that end, each account is assigned a hierarchical permission structure and each permission is assigned a pair of keys (a public key and a private key) used for signing and verification.

A smart contract is a low-level software library that contains the implementation of the actions that make a transaction. It also defines how the data that is accessed and processed by the actions are stored. A smart contract is implemented in a high-level language such as C++ and compiled into a WebAssembly (WASM) binary. Thereafter, smart contracts can be deployed and run within an account’s sandbox on the blockchain.

Incorporate the above answer in this section:
https://developers.eos.io/welcome/latest/protocol/accounts_and_permissions