Add scroll to top button

[theskumar]

Adds a scroll to top button to all the pages, it shows only if the user scroll past a decent screen size. It's put on the center top so that it's easily accessible.

Edit: Show only when the user is scrolling towards the top.

closes #3642

[frjo]

Works well.

I prefer the back button near the bottom instead the top of the window. But this is likely a personal preference and might depend on how big your screen is.

[theskumar]

Thanks for approving. I used this pattern from mkdocs-material. While the button on the right bottom is quite common, there is are couple reason for not placing it there.

1. While you scroll down, your focus in at the bottom, it cutters that space.
2. We have a sidebar with quite a lot of buttons, the right bottom will look quite busy. It will also overlap with other buttons.
3. While scrolling up, the user attention is at the top area, and often forgets to pay attention to the bottom right.
4. Another thing present on the mkdocs-material, is that it doesn't show the button while you are scrolling down, it shows only when you scrolling in upward direction. While is I find it nice, but didn't implement it here so it's more visible to users + slight simple code/performance gain to track which direction user is scrolling.

[theskumar]

Updated so it's visible while scrolling up, also adjusted a bit of styling and animation. Let me know if you would prefer this over always visible. After implementing I tend to like this version better. Thoughts?

ee3c985

[wes-otf]

@theskumar I'll be checking this out in the next hour or so. Looks like a great addition!

[frjo]

Latest version on test in a moment.

[wes-otf]

All looked and worked very good!