remove extra sections in left side nav

[tehlowkeywiz]

We're currently displaying all available sections in the left side nav, but all sections except the current one should be hidden to the user to remove as much noise as possible from the screen. The top level nav is used to navigate the sections, the left side nav is used to navigate the current section only, and the right side nav is only used as a table of content for the current page.

The side nav title is also redundant and the second one should be removed.