Split documentation to multiple files in docs/ and use markdown

docs/incompatibilities.md

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+# Backward incompatible migrations
+
+The linter analyses your migrations and checks the SQL for:
+
+- Added `NOT NULL` columns, which don't have a DEFAULT value
+- Dropping columns
+- Dropping tables
+- Renaming columns
+- Renaming tables
+- Altering columns (which can be backward compatible and potentially ignored)
+- Adding a unique constraint
+
+Those are the most important and frequent backward incompatible migrations.
+We are happy to add more if you can specify them to us.