Add collapsible capability to docs sidebar

[DunnCoding]

Is your feature request related to a problem? Please describe.
With the work being done in #257 the sidebar will be increasing in size, to the point where having all items display all the time will be an issue.

Describe the solution you'd like
Add a collapsible effect to the sidebar so users can click to expand certain sections. Clicking on an item should close the one thats currently open. This will ensure only one section can be expanded at a time allowing us to have better control over the size of the sidebar.

Describe alternatives you've considered
Enable users to be able to expand as many sections as they want and provide an 'expand/collapse all' button.

Additional context
Whilst this isn't a huge concern now, the functionality could be put in place ready for the coming increase in docs.