Docs: sidebar navigation menu covers only one section

[sadielbartholomew]

I noticed the sidebar in the documentation lists out the contents of only one section, 'Main functionalities', whereas the full documentation has numerous important sections at that top level of the contents tree, as illustrated (compare the sidebar and the Table of Contents listings) by this screenshot:

I think it would be much more helpful to readers for navigation purposes to, in the sidebar, instead list all of those top-level sections out with one or two more sub-levels of the tree for each.

(As noted when reviewing towards openjournals/joss-reviews#2407.)