Restructure user guide

Pages are now placed in a more hierarchical fashion. Let's hope this will improve the maintainability.\
Also, we can get rid of the complex UserGuide start page.
Treat FitNesse as a special word.
It's not linked, unless a page called FitNesse exists.
Corrections, mainly in GettingStarted and Wiki section.
Content updates. Mainly fixed links.
Little cleanup in FixtureGallery content.
Remove FitLibrary user guide pages.
Moved GettingStarted stuff to top level.
Fix links now GettingStarted content is on the first page.
Merge pull request #419 from amolenaar/newuserguide

Reorganized the UserGuide
Rename page PluginUsage to ConfigurationFile.