Update visual presentation
    
    This update introduces structural changes to the way we manage content
    in the website. I tried to reduce the number of changes introduced here
    as much as possible, however, and despite of that, there are still lots
    of structural changes that we need to apply in order to implement the
    gem-based model and the layouts it provides correctly. This commit
    describes such changes.
    
    - Previously, for changing the website visual presentation we change
      files in the website itself, based on minima the default theme gem
      jekyll uses. Now, changes to website visual presentation take place in
      jekyll-theme-centos gem.  Please check:
    
      * https://jekyllrb.com/docs/themes/
      * https://rubygems.org/gems/jekyll-theme-centos
      * https://gitlab.com/areguera/jekyll-theme-centos
    
      To protect the website against undesired visual presentation changes
      in jekyll-theme-centos gem, we set specific versions of
      jekyll-theme-centos gem we want to use in the Gemfile and Gemfile.lock
      files.  This way we can decouple presentation in different
      environments (e.g., development, staging and production). To update
      gems to a new version is necessary to run "bundle update" command.
      Please check:
    
      * https://jekyllrb.com/docs/step-by-step/10-deployment/
    
      The _layouts and _includes in the website directory structure were
      removed to reuse them from jekyll-theme-centos gem theme instead.  The
      information previously stored in the website _layouts and _includes
      directories was migrated to Data Files (see bellow).
    
      The assets/css directory in the website was also removed to promote
      usage of that one provided by jekyll-theme-centos.
    
      jekyll-theme-centos doens't maintain information related to CentOS
      sponsors, so images related to CentOS sponsors was preserved under
      assets/img/sponsors/ directory.
    
      File paths in the webiste directory structure replace paths in
      jekyll-theme-centos gem theme directory structure.
    
    - Previously, for maintaining entries in sections like news-and-events
      it was required to edit some specific layout files and write HTML code
      in them. This edition model was replaced by the "post" feature jekyll
      provides. So, instead of changing HTML code to maintain new entries in
      news-and-events section, you create new post files in markdown format
      under the special _posts/ directory. Please check:
    
      * https://jekyllrb.com/docs/posts/
    
      The entries related to posts in the news-and-events category are
      retained and can be subject of searching.
    
    - Previously, for maintaining the download matrix related to
      centos-linux and centos-stream distributions it was necessary to edit
      files under _layouts and _includes then write HTML code to reflect the
      change. This model was replaced by the "Data Files" feature jekyll
      provides. Now you write one YAML file and that information is taken to
      render the download matrix related to centos-linux and centos-stream
      distributions during the website building process. We also use YAML
      files to render other elements like navbar menu and footer sections.
      Please check:
    
      * https://jekyllrb.com/docs/datafiles/
      * _data/centos/distributions.yml (controls distributions data)
      * _data/centos/footer.yml (controls website footer data)
      * _data/centos/navbar.yml (controls website header navbar menu data)
    
      The intention of this change is to simplify the edition process for
      those elements and provide a more consistent experience for editors.