add vpn docs

docs/systems-administration/vpn.md

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+Connecting to the Server
+========================
+
+Access to the cstock server requires connecting to the VPN.
+Follow the steps below to connect to the VPN and access the server.
+
+## VPN set up
+
+To get set up initially ask a system administrator to set you up with a new VPN account, and two-factor application.
+You may need to install an older version of Sophos Authenticator to get the two-factor codes to work.
+
+Once you've gotten set up, download the OpenVPN config file and save it locally.
+
+## Logging in to the VPN
+
+On Ubuntu, login to the VPN by running:
+
+```bash
+sudo openvpn --config ~/user__ssl_vpn_config.ovpn
+```
+
+Enter your credentials, which are you username, followed by your password + 2FA code appended together.
+So if your password is `hunter2` and the 2-factor code is `123456`,
+you would enter `hunter2123456` for the password field.
+
+## Logging in to cstock
+
+Once on the VPN, you can access cstock by running:
+
+```bash
+ssh [email protected]
+```
+
+Replace `user` with your username, or use the `cstock` user for shared access.