add api user guides in doc

docs/content/doc/user-guides.en-us.md

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+---
+date: "2018-06-19T16:00:00+02:00"
+title: "User-guides"
+slug: "user-guides"
+weight: 45
+toc: false
+draft: false
+menu:
+  sidebar:
+    name: "User-guides"
+    weight: 45
+    identifier: "user-guides"
+---

docs/content/doc/user-guides/api-authentication.md

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+# API user guide
+
+## User
+
+### /users/{username}/tokens
+
+Basic usage:
+
+```
+$ curl --request GET --url https://m:mypassword@git.your.host/api/v1/users/m/tokens
+[{"name":"test","sha1":"..."},{"name":"dev","sha1":"..."}]
+```