Front page improvements

* Abbreviate 'What is the Web Experience Toolkit?'
  * It repeats itself, and repeats content under benefits that needn't be promoted up here
* Re-order and abbreviate 'Key resources'
  * It's not necessary to link to "Versioning", since both "Version history" and "Roadmap" link to it
   * "Contributor guidelines" did not link to any contributor guidelines...
* Add links under 'Benefits'