CLDC-2608: add documentation for the app api (#2015)

docs/adr/index.md

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 # Architecture decisions

docs/app_api.md

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+
+# Using the App API
+
+In order to use the app as an API, you will need to configure requests to the API as so:
+
+* Configure your request with Basic Auth. Set the username to be the same as `API_USER` and password as the `API_KEY` (`API_USER` and `API_KEY` are environment variables that should be set for the application)
+* Check the endpoint you are calling is an action that is `create`, `show` or `update`
+* Check you are setting the following request headers:
+  * `Content-Type = application/json`
+  * `Action = application/json` N.B. If you use `*/*` instead, the request won't be recognised as an API request`
+
+Currently only the logs controller is configured to accept and authenticate API requests, when the above API environment variables are set.

docs/documentation_website.md

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 # This documentation website

docs/form/index.md

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 # Generating forms

docs/setup.md

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 # Local development