Add basically API documentation

docs/api/admin.md

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+# /admin
+
+This endpoint is for the server admin.
+
+Due to that, it requires authentication using the HTTP basic authentication.
+The username can be anything, the password must be the configured admin token.
+
+Additionally, this endpoint allows cross origin requests.
+
+## GET /admin/status
+
+Use this to get the server status.
+
+## Response
+
+This returns a JSON object with ``websocketClients`` (of the type number,
+the number of clients connected using the websocket) and the map ``counters``
+which maps values to numbers. You should not make any assumptions about the counter names
+and their availability.
+
+### example response
+
+```
+{
+  "websocketClients": 3,
+  "counters": {
+    "testCounter": 1
+  }
+}
+```
+
+## POST /admin/reset-counters
+
+Use this to reset the counters included in the server status.
+
+Although this uses POST, it does not take any request body
+
+Response: ``{"ok": true}``
+
+## GET /admin/status-message
+
+Use this to get the current status message.
+
+This returns a object with the key ``statusMessage`` whose value
+of the type string is the current status message.
+
+Example response: ``{"statusMessage":""}``
+
+### see
+
+- [status message concept](../concept/status-message.md)
+
+## POST /admin/status-message
+
+Use this to set the status message.
+
+Request body: object with ``message`` (string)
+
+Response: ``{"ok": true}``
+
+### see
+
+- [status message concept](../concept/status-message.md)
+
+## POST /admin/unlock-premium
+
+Use this to unlock all features for one user for a specified duration.
+
+### request
+
+request properties: ``mail`` and ``duration``
+
+- ``duration`` must be ``year`` or ``month``
+- ``mail`` must be a mail address of any user assigned to the family for which the features should be unlocked
+
+### response
+
+If everything worked:
+
+``{"ok": true}``
+
+If the duration was invalid or no mail address was specified: HTTP status code 400 Bad Request
+
+If there was nothing found for the mail address: HTTP status code 409 Conflict
+
+### see
+
+- [premium concept](../concept/premium.md)