Improve time zone document (#714)

- Move from Common Tasks to Configuration Guides, since most users will
  just use the GUI

- Rename from `change-timezone-terminal.md` to `change-time-zone.md`
  (1) because we don't want multiple time zone documents for terminal
  and non-terminal usage and (2) to make the orthography of "time zone"
  consistent; update title and permalink accordingly

- Make heading syntax consistent

- Add heading

- Fix code block syntax

- Fix typo

- Add page to ToC

configuration/change-time-zone.md

@@ -0,0 +1,36 @@
+---
+layout: doc
+title: Changing your Time Zone
+permalink: /doc/change-time-zone/
+---
+
+# Changing your Time Zone #
+
+## Qubes 4.0 ##
+
+### Command line ###
+
+If you use the i3 window manager or would prefer to change the system's time
+zone in terminal you can issue the `timedatectl` command with the option
+`set-timezone`.
+
+For example, to set the system's time zone to Berlin, Germany type in a dom0
+terminal:
+
+    $ sudo timedatectl set-timezone 'Europe/Berlin'
+
+You can list the available time zones with the option `list-timezones` and show
+the current settings of the system clock and time zone with option `status`.
+
+Example output status of `timedatectl` on a system with time zone set to
+Europe/Berlin:
+
+    [user@dom0 ~]$ timedatectl status
+          Local time: Sun 2018-10-14 06:20:00 CEST
+      Universal time: Sun 2018-10-14 04:20:00 UTC
+            RTC time: Sun 2018-10-14 04:20:00
+           Time zone: Europe/Berlin (CEST, +0200)
+     Network time on: no
+    NTP synchronized: no
+     RTC in local TZ: no
+