Document how to set up a Jenkins build server

Author: Tom-Willemsen

doc/systems/Build-Servers.md

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+# Build Servers
+
+In order to set up a new build server, capable of running GUI, Uktena, and EPICS builds, you need to install:
+- Visual Studio
+- Perl
+- Java
+- Maven
+- Doxygen
+- git
+- TwinCAT bits
+
+## Connecting to Jenkins
+
+Use the given URL _without_ the `-websocket` flag.
+
+The node needs adding in Jenkins, configuring with a number of executors, and labels `epics_build`, `gui`, `genie` adding to it.