(docs) add system_accounts config value

    ]

```

## System Accounts

- Config option containing important system account usernames

  - To get system accounts: `occ config:system:get ecloud-accounts.system_accounts`

  - To set first system account username: `occ config:system:set ecloud-accounts.system_accounts 0 --value=username`

  - To use in code `$systemAccounts = $this->config->getSystemValue('ecloud-accounts.system_accounts', []);`

## User Account creation

- This plugin creates an endpoint `/apps/ecloud-accounts/api/set_account_data` that is to be used to set user's email, quota,recovery email and create the user's folder if necessary