This is a Ghidra extension that helps you manage Ghidra extensions. Available extensions information is pulled from the Ghidra Extension Manager Repository.
Go to the releases page and download the latest version for your Ghidra distribution.
In Ghidra main window go to File > Install extensions.... In the new window press the + icon to import the downloaded zip.
Once the script is installed, in the main program window you will be able to open the manager by going to File > Extension manager....
For development instructions checkout doc/Develop.md.