Documentation needs to be reworked

The documentation has numerous issues:

• C++ documentation is not generated, there are lots of warnings about classes not found
  • since Qt 5.11 qdoc uses clang to parse sources, so this might be missing includes, macros causing this
• QML documentation has no import statements, could be linked to above issue
• documentation is installed in /usr/share/u1db-qt/doc, does not use DOCDIR from the GNUInstallDirs
• uses obsolete markup like \chapter, \part which has been removed from qdoc
• Ubuntu namespace needs to be renamed to Lomiri
• Google fonts usage should be removed
• there are URLs pointing to Ubuntu/Launchpad
• example 6 contains a verbatim copy of the documentation from example 5 which has nothing to do with it
• examples documentation is empty and linked from the main documentation

CC @sunweaver