We recently went down this rabbit hole. We extensively tested each of the following:

- http://notion.so/

- https://slab.com/

- https://getoutline.com/

- https://gitbook.com/

Each has its strengths and weaknesses. Notion is the most flexible and fun to use. Outline and Gitbook are open source, which is nice. Slab is probably the furthest along in development and in terms of being purpose-built for knowledge management.

Unfortunately, Google Sites seems to be super low priority at Google.