[#153] Add footer to website templates

[shamshaev]

I checked once again overall application style and propose to improve it a little bit. Now the contrast is too high between navbar, content and footer. It's dark-light-dark now. Usually dark part only one - navbar or footer. So, I propose to make footer light. I think it will look better. By the way, this color scheme is in the file "landing.html", I just didn't implemented it fully when I did previous PR.

Current color scheme:

Proposed color scheme (from "landing.html"):

Demo: https://hexlet-correction-4.onrender.com/

[Malcom1986]

@fey Коля, посмотри плиз, как это выглядеть будет

[fey]

вроде норм @Malcom1986 если ок, мержи)