Update TIOBE paragraph on about page (de)
Add Ruby Toolbox paragraph to libraries page (de)
Move "Other Implementations" to about page (de)

Move section from downloads page to about page,
slightly adapt its first paragraph to the new context,
introduce named link references.
Move installation steps to installation page (de)
Merge pull request #634 from stomar/update-de

Update downloads, about, installation, and libraries page (de)