Rename Drupal 8 to Drupal X

Issue Summary

There is currently no available release of Drupal 8 on the core project page. This is because Drupal 8 does not exist. However, there exist API documentation and discussions referring to Drupal 8. What they instead refer to is Drupal X, the experimental version of Drupal. (Literally, a version of Drupal that is being used to experiment upon.)

As reported, the official references to Drupal 8 imply to new users that they must make a choice between using Drupal 8 over Drupal 7. This choice does not exist, as Drupal 8 does not exist. There is no choice between using a full version and an experimental build; the intention of using a full version is to produce a website, the intention of using an experimental version is to produce a full version.

Eventually, the full version Drupal 8 alpha will be branched off the experimental version by Dries. Once that arrives, new users will have a real choice between Drupal 7 stable and 8 alpha, and the tradeoffs of that choice will be clear to them.