Update documentation for new AttributeType system

- Rewrite customtype.md with comprehensive documentation:
  - Overview of encode/decode pattern
  - Required components (type_name, dtype, encode, decode)
  - Type registration with @dj.register_type decorator
  - Validation with validate() method
  - Storage types (dtype options)
  - Type chaining for composable types
  - Key parameter for context-aware encoding
  - Entry point packages for distribution
  - Complete neuroscience example
  - Migration guide from AttributeAdapter
  - Best practices

- Update attributes.md to reference custom types

Author: claude

docs/src/design/tables/attributes.md

@@ -77,6 +77,10 @@ sending/receiving an opaque data file to/from a DataJoint pipeline.
 - `filepath@store`: a [filepath](filepath.md) used to link non-DataJoint managed files
 into a DataJoint pipeline.
 
+- `<custom_type>`: a [custom attribute type](customtype.md) that defines bidirectional
+conversion between Python objects and database storage formats. Use this to store
+complex data types like graphs, domain-specific objects, or custom data structures.
+
 ## Numeric type aliases
 
 DataJoint provides convenient type aliases that map to standard MySQL numeric types.