Sticky top bar/hamburger icon in mobile view

**Your Godot version:**
This pertains to the documentation website itself.

**Issue description:**
The current website converts the fixed sidebar menu to a top bar with a toggleable sidebar. This behavior itself is great - the problem is that the top bar is actually at the top of the *whole page* instead of just the screen.

This means that if I'm reading a documentation page and want to access anything in the side bar, I have to manually scroll all the way back up to the top, making the website several times more frustrating to use on mobile.

As an added point of frustration to this behavior, "pulling down" when at the top of the page is a standard mobile browser shortcut for reloading the page - and since I was often far down the page due to clicking an anchor link, this means if I'm not super careful about scrolling up, I'll accidentally reload and be jumped allllllllll the way back down.

If the top bar was stickied at the top of the view instead, it would completely solve this issue.