Change MIGRATION.md to Docker instructions

This old guide was recommending the use of manual `pg_dump`s, which are much easier to get right when you have the admin section backups do it for you. It felt like it was actively harmful to keep around (I've seen a few people on Meta trying to do `pg_dump`s), so I replaced the content with https://meta.discourse.org/t/migrating-an-old-discourse-install-to-docker/12439 .