Offline-documentation cleanup

This patchset cleans up the offline documentation:

 * Update old references to svn with references to git

 * Remove direct links to online resources - Those do not belong into the
   offline documentation, and are already easily available on the homepage.

 * Remove stub content that only pointed to online location

 * Unify indentation stuff
Well, all of this external links should be accessible from the homepage, i.e. under http://www.dealii.org -> [Dev] -> ..., The changelog is not, the rest of the links are.

I really think that we should use the opportunity to clearly streamline documentation - and the new homepage does this beautifully. Having those links in multiple places has the problem to increase maintenance, and is irritating.