Context:

Once the external variable API topic has been implemented, we should add the relevant documentation.

AT:

• New external variable API is documented
• New external variable config is also documented
• Docs include clear warning that ingesting variables which also occur in the engine will lead to inaccurate reports
• Docs mention that the API may change in the future
• Docs explain importance of accurate instance information (eg incorrect instanceID or defKey will lead to new instances and instance indices that wont be available in reports)
• Docs state that variables must belong to instances originating in the engine (and not eg event based instances)
• Docs mention that variable plugin will not affect ingested variables