Minor fixes (broken links, missing images)

[kerhac]

I have found some minor issues and fixed them. First I noticed the contributing link in the German translation was running into a 404. Then I ran linkchecker to find the rest.

Changes in this pull request:

• Add missing images taken from English translation \ This will add images which are reference to in the markdown files but are not present. The files are copies from the English translation. I would guess it's better to have English pictures than none.
• Fix broken links due to html reference \ This will replace the wrong reference to .html files with .md files. The static page generator will take care of correctly replacing the file extensions while generating the page.
• Fix broken links in installation documentation
• Fix link to CONTRIBUTING \ The contributing guidelines are located in the main README.md and not in a separate file. This will align the German translation with the English one.

[ekohl]

I merged a PR that introduced a merge conflict. Could you update it?

[kerhac]

I have rebase onto master and dropped the commit fc3748a10 Fix broken links in installation documentation since the whole text block has been removed in 2e461459a.