A few enhancements for the docs website

[jwortmann]

• logo in navigation bar
• favicon
• scroll-to-top button because some of the pages are very long
• copy button for code blocks
• add filenames to some of the code blocks
• fancy tabbed pane for the setting comparison preview images for the code lens feature

I'd like to replace the entire "Features" page with one two bigger screenshots that show details when you hover with the mouse, like on the https://www.julia-vscode.org/ website. They use jQuery and https://github.com/jpchateau/Interactive-Image for it though, and I'm not sure if this is possible to do in Markdown / with mkdocs. Also some newer features implemented in LSP were not yet added to the docs. And in general the text in the existing page could need a revise; I guess nobody really needs an explanation what "auto-complete" is, for example.

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