mullvad: add page (#3793)

* Add mullvad.md

Information on the mullvad command line utility.

* Update mullvad.md

* Update pages/linux/mullvad.md

Co-Authored-By: Zlatan Vasović <zlatanvasovic@gmail.com>

* Update pages/linux/mullvad.md

Co-Authored-By: Zlatan Vasović <zlatanvasovic@gmail.com>

* Update mullvad.md

Co-authored-by: Zlatan Vasović <zlatanvasovic@gmail.com>

pages/linux/mullvad.md

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+# mullvad
+
+> CLI client for Mullvad VPN.
+> More information: <https://mullvad.net/>.
+
+- Link your mullvad account with the specified account number:
+
+`mullvad account set {{account_number}}`
+
+- Enable LAN access while VPN is on:
+
+`mullvad lan set allow`
+
+- Command the client to start establishing VPN tunnel:
+
+`mullvad connect`
+
+- Check status of VPN tunnel:
+
+`mullvad status`