Integrate overview pages with their parent entry

[szczys]

Current behavior of the right sidebar navigation doesn't load page content when clicking on folder-level entries.

Reproduce:

• Go to https://docs.golioth.io/hardware
• Click on Virtual Devices
• The menu unfolds but page content in the main pane doesn't change to the new topic

Expected behavior:
Clicking anything on the left sidebar menu loads a new page

Suggested fix:
Make overview pages for that section load when folder-level entries are clicked.

This would have the added benefit of cleaning up the sidebar. The ESP32 and nRF91 shown below don't need both Overview and Quickstart. They could just have the ESP32 show the overview and all of the quickstart items would be subheadings of that:

[szczys]

This is been taken care of throughout the repo over the past two years.