Add close button to suggested reading popups

[jdpigeon]

I notice these suggested reading popups in many places across the site

They're nice, but the inner neat-freak in me wants to be able to close them.

I see them defined in the _layouts .html files. It shouldn't be too hard to add a close button

I'll look into this and make a PR

[ahmetcecen]

The sidebar content was hastily designed and can certainly use many improvements. Feel free to improve on it.

Note that the sidebar content will only be visible in screens with a resolution close to 1080p at the moment. This is probably another area of improvement, like stacking them below the page etc.