Update for new API (#5654)

Update for new fixer.io API

source/_components/sensor.fixer.markdown

@@ -18,16 +18,27 @@ The `fixer` sensor will show you the current exchange rate from [Fixer.io](http:
 
 To get an overview about the available [currencies](http://api.fixer.io/latest).
 
+## {% linkable_title Setup %}
+
+You need to create an [API key](https://fixer.io/product). There is a rate limit of 1000 calls per month.
+
+## {% linkable_title Configuration %}
+
 To enable this sensor, add the following lines to your `configuration.yaml` file:
 
 ```yaml
 # Example configuration.yaml entry
 sensor:
   - platform: fixer
+    api_key: YOUR_API_KEY
     target: CHF
 ```
 
 {% configuration %}
+api_key:
+  description: Your API key for [Fixer.io](http://fixer.io/).
+  required: true
+  type: string
 target:
   description: The symbol of the target currency.
   required: true
@@ -37,9 +48,6 @@ name:
   required: false
   type: string
   default: Exchange rate
-base:
-  description: The symbol of the base currency.
-  required: false
-  type: string
-  default: USD
 {% endconfiguration %}
+
+Details about the API are available in the [Fixer.io documentation](https://fixer.io/documentation).