Update Flutter Tutorials section on the docs site

[Rexios80]

These tutorials need updated to reflect the best practices of Hive CE such as immutable data models and using the GenerateAdapters annotation

• https://github.com/IO-Design-Team/hive_ce_docs/tree/master/tutorials
• https://github.com/IO-Design-Team/hive_ce_samples

For now these pages are hidden to prevent the proliferation of bad code