Add interactive training cookbook and refresh managed fine-tuning examples

[John Wu (johnwu0604)]

Summary

• Add the interactive training cookbook, training recipes, SDK guidance, and supporting documentation.
• Reorganize managed fine-tuning examples, datasets, and notebooks into the updated learning paths.
• Refresh shared documentation, tests, and repository guidance for both training tracks.
• Add explicit access-approval notices and the preview sign-up link immediately below the introductions in both READMEs.