Change documentation URLs

* Now the main documentation URL is simply /docs. That will
  redirect to /docs/<VERSIONNUMBER> and contains both the
  reference documentation and tutorials.
* Right now /docs points to /docs/0.3, but it will point to
  /docs/0.4 when 0.4 is released. All URLs for the previous
  versions will keep working, though.
* The tutorial lives now under /docs/<VERSION>/tutorial.
  /docs/tutorial exists as a convenience, pointing to the basic
  tutorial of the latest version.
* The downside is that when a new version is released, there are
  several files that have to be updated. Oh well.
* While at it, make a small refactoring for the CSS used in the
  warning and info messages, and fix a problem when those messages
  don't span several lines.