Update README.md to change section header and add new resource link.

[catatsuy]

This pull request includes a small change to the README.md file. The change adjusts the section heading and adds a new example to the case studies section.

Changes to the README.md file:

• Changed the heading from ### 事例集 to ## 事例集 to adjust the section level.
• Added a new case study example with a link to Progate Path and a brief description of its use in new graduate training.