doc backport for getting started - "Showing Posts" section. [ci skip]

Closes #13619.
Merge pull request #13355 from dylanahsmith/migration-version

activerecord: Initialize Migration with version from MigrationProxy.

Conflicts:
	activerecord/CHANGELOG.md

Conflicts:
	activerecord/CHANGELOG.md
Merge pull request #13597 from prathamesh-sonpatki/hstore_migration

Make change_table use object of current database adapter
Conflicts:
	activerecord/CHANGELOG.md
	activerecord/test/cases/adapters/postgresql/hstore_test.rb
Merge branch '4-0-stable' of https://github.com/rails/rails into v4.0.0_sqlserver