Add Codec documentation with plugin specification

- Add codec-spec.md: detailed API specification for creating codecs
- Add codecs.md: user guide with examples (replaces customtype.md)
- Remove customtype.md (replaced by codecs.md)

Documentation covers:
- Codec base class and required methods
- Auto-registration via __init_subclass__
- Codec composition/chaining
- Plugin system via entry points
- Built-in codecs (blob, hash, object, attach, filepath)
- Complete examples for neuroscience workflows

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: dimitri-yatsenko