Add some how to content, written or video. Your documentation is OK, but most for you links when looking at Elasticseach, clustering and Mongo db are just to the technologies guides on there sites. There no context on how is works with Graylog and why you need to set it up in a certain way.
This will grow the community and the project itself. A good example of this is Zabbix. They have put a lot of effect into video and written content, the community is helpful and knowledgeable, which is making them very popular with admins, which is what you want.