Add .readthedocs.yml

[c-bata]

Contributor License Agreement

This repository (optuna-dashboard) and Goptuna share common code.
This pull request may therefore be ported to Goptuna.
Make sure that you understand the consequences concerning licenses and check the box below if you accept the term before creating this pull request.

• I agree this patch may be ported to Goptuna by other Goptuna contributors.

Reference Issues/PRs

Refs #555

What does this implement/fix? Explain your changes.

Fix following errors:

The configuration file required to build documentation is missing from your project. Add a configuration file to your project to make it build successfully. Read more at https://docs.readthedocs.io/en/stable/config-file/v2.html