link to eth guest regulations

account/eth_guest_accounts.markdown

+ETH Guest Accounts
+==================
+
+Please refer to the official documentation regarding the rules of ETH guest accounts.
+
+  * [ETH guests overview](https://ethz.ch/staffnet/en/employment-and-work/guests.html)
+  * [ETH guest categories](https://ethz.ch/staffnet/en/employment-and-work/guests/guest-categories.html)
+  * [ETH guest authorisations](https://ethz.ch/staffnet/en/employment-and-work/guests/general-overview-of-authorisations.html)
+    - in particular the PDF linked on the bottom contains a matrix with the list of allowed services for each guest category