Zonemaster-GUI documents

* An overview of the Zonemaster GUI is found in [Zonemaster UI]
* Information related to code testing is found in [Unit testing Zonemaster-GUI]
* [Installation instructions]
* [Configuration instructions]
* [Translation instructions]


[Configuration instructions]:                     https://github.com/zonemaster/zonemaster/blob/master/docs/public/configuration/gui/README.md
[Installation instructions]:                      https://doc.zonemaster.net/latest/installation/zonemaster-gui.html
[Unit testing Zonemaster-GUI]:                    Testing.md
[Translation instructions]:                       https://github.com/zonemaster/zonemaster/blob/master/docs/public/translation/Translating-GUI.md 
[Zonemaster UI]:                                  UI.md