[ADF-1586] Added Core services docs (#2582)

docs/storage.service.md

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+# Storage service
+
+Stores items in the form of key-value pairs.
+
+## Methods
+
+`getItem(key: string): string | null` <br/>
+Gets an item identified by `key`.
+
+`setItem(key: string, data: string)` <br/>
+Stores an item under `key`.
+
+`clear()` <br/>
+Removes all currently stored items.
+
+`removeItem(key: string)` <br/>
+Removes the item identified by `key`.
+
+`hasItem(key: string): boolean` <br/>
+Is any item currently stored under `key`?
+
+## Details
+
+The service will check to see if
+[web storage](https://developer.mozilla.org/en-US/docs/Web/API/Storage/LocalStorage)
+is available on the browser. If it is available then the service will use it to
+store the key-value items persistently. Web storage can be used in a similar way to
+[cookies](cookie.service.md) but with a much higher size limit (several MB for
+web storage compared to a few KB for cookies). However, cookies are
+more widely supported by browsers and can be set to expire after a certain date.
+
+If local storage is not available then non-persistent memory storage within the app is
+used instead.
+
+<!-- Don't edit the See also section. Edit seeAlsoGraph.json and run config/generateSeeAlso.js -->
+<!-- seealso start -->
+## See also
+
+- [Cookie service](cookie.service.md)
+<!-- seealso end -->