Import examples from `optuna/optuna-dashboard`

[toshihikoyanase]

Motivation

Currently, the examples for Optuna Dashboard are hosted in two different repository: this repository and optuna/optuna-dashboard. Unifying them would simplify discovery for users, as all examples would be accessible from a single location.

Description of the changes

• Import dashboard examples from optuna/optuna-dashboard to ./dashboard
• I'll create a subsequent PR to replace the example files in optuna/optuna-dashboard with links to the corresponding files in this repository.

Discussion

• Should we preserve the commit history of the examples when importing them?

[toshihikoyanase]

Thanks for your comment. I agree that adding a README file to ./dashboard would be beneficial for guiding users. As you suggested, we can address this in subsequent PRs to maintain simplicity and clarity in our tracking history. Would that approach be okay with you?