Widen and scale main page

[Bluenix2]

Currently the main content is given very little room, additionally it doesn't scale very well as the window resizes. A nice balance needs to be found where the main content is given enough room to be comfortably readable, while keeping the side-navigation reasonably in the center of the screen.

See Using media queries for how to adjust the different margins depending on screen size as well as the Visual Studio Code docs and Material for MKDocs for appropriate examples.