This collection of documentation is a work-in-progress to document the Migrate API and Migrate Drupal modules for the Drupal site administrator.
The migrate module and migrate_drupal modules are now in core. They replace the upgrade.php path for moving between major versions of Drupal. The update.php page will now only be used for performing upgrades within the same version (8.x.x).
Currently only D6->D8 mappings are in the migrate_drupal module (which is a rough equivalent to migrate_d2d in the D7 contrib space). Work is being done on D7->D8 mappings and eventually D8->D8 mappings will also be developed to facilitate rescue projects and migrating to a new code base within the same version.
If you're interested in getting involved, find us in IRC in #drupal-migrate, join https://groups.drupal.org/imp and participate in the weekly G+ hangout which is documented on the groups.drupal.org page.
Developer documentation is available for writing your own custom migrations.
The migrate_upgrade module is a contrib solution that may be rolled into core in the 8.1.x branch to replicate the old "upgrade" process at /upgrade in your D8 site (rather than upgrade.php).
The migrate_upgrade module will run all available migrations for the version of Drupal you are migrating from with the idea of re-creating the whole site. If this is not what you want, you will need to create a custom manifest.yml file and use drush to run specific migrations rather than all-at-once.