message: Clarify installation, utilities and examples generation
  Add a link to the Hello GUDHI World project
diff: * \tableofcontents
  * As GUDHI is a header only library, there is no need to install the library.
  *
  - * Examples of GUDHI headers inclusion can be found in \ref demos.
  + * Examples of GUDHI headers inclusion can be found in \ref utilities.
  *
  * \section compiling Compiling
  * The library uses c++11 and requires <a target="_blank" href="http://www.boost.org/">Boost</a> &ge; 1.48.0
  * and <a target="_blank" href="https://www.cmake.org/">CMake</a> &ge; 3.1.
  * It is a multi-platform library and compiles on Linux, Mac OSX and Visual Studio 2015.
  *
  - * \subsection demos Demos and examples
  - * To build the demos and examples, run the following commands in a terminal:
  + * \subsection utilities Utilities and examples
  + * To build the utilities, run the following commands in a terminal:
  \verbatim cd /path-to-gudhi/
  mkdir build
  cd build/
  cmake ..
  make \endverbatim
  - * A list of examples is available <a href="examples.html">here</a>.
  + * By default, examples are disabled. You can activate their compilation with
  + * <a href="https://cmake.org/cmake/help/v3.0/manual/ccmake.1.html">ccmake</a> (on Linux and Mac OSX),
  + * <a href="https://cmake.org/cmake/help/v3.0/manual/cmake-gui.1.html">cmake-gui</a> (on Windows) or y mofifying the
  + * cmake command as follows :
  +\verbatim cmake -DWITH_GUDHI_EXAMPLE=ON ..
  +make \endverbatim
  + * A list of utilities and examples is available <a href="examples.html">here</a>.
  + *
  + * \subsection libraryinstallation Installation
  + * To install the library (headers and activated utilities), run the following command in a terminal:
  + * \verbatim make install \endverbatim
  + * This action may require to be in the sudoer or administrator of the machine in function of the operating system and
  + * of <a href="https://cmake.org/cmake/help/v3.0/variable/CMAKE_INSTALL_PREFIX.html">CMAKE_INSTALL_PREFIX</a>.
  *
  * \subsection testsuites Test suites
  * To test your build, run the following command in a terminal:
  @@ -30,6 +42,10 @@ make doxygen
  # Documentation will be generated in the folder YYYY-MM-DD-hh-mm-ss_GUDHI_X.Y.Z/doc/html/
  # You can customize the directory name by calling `cmake -DUSER_VERSION_DIR=/my/custom/folder`
  \endverbatim
  + *
  + * \subsection helloworld Hello world !
  + * The <a target="_blank" href="https://gitlab.inria.fr/GUDHI/hello-gudhi-world">Hello world for GUDHI</a>
  + * project is an example to help developers to make their own C++ project on top of the GUDHI library.
  *
  * \section optionallibrary Optional third-party library
  * \subsection gmp GMP
  