Add possibility to navigate in a more flexible way through the documentation

[nbro]

So far, from what I understood, we can just go forward by clicking on the sub-modules or sub-packages' names on the left side panel, but we can't go back to parent modules or packages:

unless we click to go back in the browser.

It would be nice to have a more flexible way of navigating through the documentation.

[cudmore]

I agree. Maybe the left menu pane (index in the above example image) could provide constant links to all sub-modules? Otherwise, all is good.