Improve the UI for using Server Capabilities as variables

In order to know which variable name to use for an executable, one must click on that executable from Server Capabilities and check the last part of the URL. For example, after adding a Command capability, and clicking on that capability, the last part of the URL is: "capabilityKey=system.builder.command.test". The correct Variable name for accessing the value of that executable is: ${bamboo.capability.system.builder.command.test}

This is confusing, and it would be great to improve the UI to indicate this information.

After improving the UI, we also need to improve the documentation that corresponds to this part