Add outline of `migration.md` page

migration.md

+# Data migrations
+
+The old new website of Study Association Thalia contained a lot of data.
+Before we can launch the new website, all old data has to be migrated
+to the new website.
+The migration instructions for each part of the website are shown below.
+
+## Active members
+
+## Documents
+
+## Education
+
+## Events
+
+## Mailing lists
+
+## Members
+
+## Newsletters
+
+## Partners
+
+## Photos
+
+## Pizzas
+
+## Thabloids
+
+## Merchandise