Navigation tree depth is limited

[Rbiessy]

The depth of the navigation tree on the left seems to be limited to 3 levels. The sparse blas domain could use a 4th level so that the functions under the "Data handles" page are more easily accessible.
The html page does not allow to expand the "Data handles" page but my local html build does, see the image below and the html build: docs.zip

The issue was partially discussed in #588 and the PR #597 was not enough to fix it.