Issue #2845486 by Jo Fitzgerald, quietone, gaurav.kapoor, ohthehugemanatee, heddn, Yogesh Pawar, catch, shabana.navas, phenaproxima, mikeryan: Rename Migration process plugin and add documentation

Revert "Issue #2560795 by mikeryan, phenaproxima, penyaskito, willwh, benjy: Source plugins have a hidden dependency on migrate_drupal"

This reverts commit 89815ba3.

Issue #2560795 by mikeryan, phenaproxima, penyaskito, willwh, benjy: Source plugins have a hidden dependency on migrate_drupal

Issue #2625696 by chx, benjy, alexpott, catch, mikeryan, andypost, phenaproxima: Make migrations themselves plugins instead of config entities

Issue #2534012 by phenaproxima, hussainweb, mikeryan: Move module-specific migration support into the file module

Issue #2463909 by phenaproxima, mikeryan: Migrations should support non-installed default configurations (templates)

Issue #2460529 by alexpott, chx, phenaproxima: Migrations need to use the configuration entity dependency system

Issue #2466797 by mikeryan, benjy: Rename migration_groups key to avoid confusion with MigrationGroup support

Issue #2121299 by andypost, bdone, benjy, penyaskito, chx, claudiu.cristea, damiankloip, gregboggs, InternetDevels, jessehs, jhedstrom, marvil07, mikeryan, pcambra, Xano, YesCT, axoplasm, kgoel, dclavain, jeckman, alvar0hurtad0, Drupali, fastangel, gloob, jhedstrom, joshtaylor, mpgeek, alexpott: Migrate Drupal 6 to Drupal 8