Use pre elements for code

This allows us to group the commands to be entered one after another.
This avoids leaving a gap between the lines -- the p elements had an
extra bottom margin that caused the white background to bleed through.

The lines are still wrapped when the window is too narrow to make them
fit -- as they did before in the p element.
css: add border and padding to code examples

Now that we have grouped related commands together in a pre element,
we can give them a nice border and some padding to avoid a too dense
look.
Merge pull request #2 from mgeisler/code-color

Improve code blocks