create PR template

need a generic PR template with a detailed description, motivation, test info, and backward compatibility details in it

Look for existing PR templates in the repository's .github folder or similar locations. The template should include sections for description, motivation, testing information, and backward compatibility. Check if there are any contributing guidelines to align with. Once created, verify the template works by opening a dummy PR.

索引モデルが issue の本文から書いたものです。