Write documentation for Drupal 8

As a part of #2533684: Create 'Documentation' Section we started migrating documentation into the new system: #2762837: Migrate documentation into the new system. While doing that we are also splitting documentation per major version D7/D8 #2587331: Split documentation per Drupal version.

Most of the existing documentation will be about Drupal 7. When Drupal 8 content already exists, we will split it from D7 content and move into new D8 section of documentation. However, for a lot of documentation there is no D8 version of content, hence it needs to be written.

Issues for individual guides:
#2774779: Create Choosing Drupal version guide for Drupal 8
#2774765: Create Understanding Drupal guide for Drupal 8