Update homepage section around centos
    
    - The "Around CentOS" section in the homepage was built using the
      jquery.rss.js. Recently we detected the content was not any longer
      visible and checking the script found it was consuming an external
      site (http://www.feedrapp.info/) that is currently out of service. As
      consequence the "Around CentOS" section in the homepage shows no
      information.
    
      This update converts the file "assets/planet.rss" into
      "_data/centos/planet.yml" and updates the section code to read this
      file instead. The jquery.rss.js file was removed and the approach it
      provides no longer considered. So to prevent this from happening
      again.
    
      A future commit must remove the planet.yml provided in this commit, so
      another one, automatically generated take its place. This way updates
      from planet can be visible in the home page.
    
      For the RSS to YAML conversion used the following python script:
    
      ----
      import xmlplain
    
      # Read to plain object
      with open("assets/planet.rss") as inf:
        root = xmlplain.xml_to_obj(inf, strip_space=True, fold_dict=True)
    
      # Output plain YAML
      with open("_data/centos/planet.yml", "w") as outf:
        xmlplain.obj_to_yaml(root, outf)
      ----