Make code content and tables more responsive

* 'pre' blocks and tables use scrollbars if the viewport is too narrow
* Code content which is not within a 'pre' element wraps automatically
* Code content and 'pre' blocks have a gray background color

The added background color has two purposes:
* The left-margin for pre blocks is removed, because it consumes too
  much space on narrow viewports. However by using a background color,
  the reader can still recognize 'pre' content as code blocks.
* Code content which is *not* within a 'pre' element wraps
  automatically now. The background color allows the reader to
  recognize the "connection" between automatically splitted lines of
  a single code element. This way, nobody will mistake a one-liner for
  a multi-liner.

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