Work on documentation

  modified:   doc/applications.rst
  modified:   doc/index.rst
  modified:   doc/install.rst
  modified:   doc/installation/clang.rst
  modified:   doc/installation/install_options.rst
  modified:   doc/installation/requirements.rst
  modified:   doc/installation/ubuntu.rst
  new file:   doc/changelog.rst
  new file:   doc/versions.rst