Add a button to collapse the navigation sidebar

**Your Godot version:**
The UI of the Godot Docs page. So all versions?

**Issue description:**
A button to collapse the navigation sidebar on the Godot docs webpage is needed. When the docs browser page is snapped to half the screen on my desktop, the sidebar obstructs the view. A button, styled as `<` in a circle, could be placed along the border of the sidebar for manual collapsing. Once collapsed, the button would flip to a `>` in a half-circle, allowing users to click it again to expand the sidebar as needed. It can sit pegged beside the Godot Docs logo.

The design is just a suggestion; I care more about the UI performing its purpose. Overall, the feature would be greatly appreciated.

**URL to the documentation page (if already existing):**
N/A