Enable collapsible sidebar sections

Currently the Guidebook sidebar sections aren't collapsible.

This means a lot of scrolling is required in denser content tabs ("areas") like How-to.

By removing or commenting out theme.features.navigation.sections in the MkDocs configuration here, we enable collapsible sidebar sections.

This makes it easier to see at-a-glance all the categories, and drill into only what's relevant, without excess scrolling.

It also facilitates purview of categories/sections to enable better (re-)organisation moving forward, and help avoid duplication or confusion for where new content goes.