[ADF-4152] Updated folder structure of core docs (#4415)

* [ADF-4152] Moved core library docs into subfolders

* [ADF-4152] Moved core library docs into subfolders

* [ADF-4152] Manual fixes to core doc file links

* [ADF-4152] Further automatic + manual link tidying

docs/core/services/jwt-helper.service.md

@@ -0,0 +1,29 @@
+---
+Title: JWT helper service
+Added: v3.0.0
+Status: Active
+Last reviewed: 2019-01-09
+---
+
+# [JWT helper service](../../../lib/core/services/jwt-helper.service.ts "Defined in jwt-helper.service.ts")
+
+Decodes a JSON Web Token (JWT) to a JavaScript object. 
+
+## Class members
+
+### Methods
+
+-   **decodeToken**(token: `any`): `Object`<br/>
+    Decodes a JSON web token into a JS object.
+    -   _token:_ `any`  - Token in encoded form
+    -   **Returns** `Object` - Decoded token data object
+
+## Details
+
+JWT is a standard for sending data securely that ADF uses during the
+OAuth2 authentication procedure. See the [JWT website](https://jwt.io/)
+for full details of the standard and its uses.
+
+## See also
+
+-   [Identity user service](identity-user.service.md)