Merge branch 'bugfix/remove-image-floating' into 'master'

ref:infrastructure/dune-website This 'fixes' the problem of the image not
showing up so it is an improvement. I'm not sure why the floating environment
used to work (or still works) for the Dune 2.3 release. Building the dune
website locally behaves strange at my machine (see comment in [#74]) so I
didn't investigate further.

If someone comes up with a real solution feel free to update this merge
request (or close it) :)

See merge request [!253]

  [#74]: gitlab.dune-project.org/NoneNone/issues/74
  [!253]: gitlab.dune-project.org/infrastructure/dune-website/merge_requests/253