Docs: Update issue and PR templates

[nick-y-ito]

Overview

This PR updates the issue templates and pull request templates since the current ones do not fit our needs.

Changes

• Deleted all the existing issue templates
• Added the following three issue templates
  • enhancement.md: For new features, ideas or any other enhancement
  • problem.md: For bugs or any other problems
  • task.md: For the tasks our team members would take charge of
• Added a new PR template called merge.md used when merging the develop branch to the main branch
• Changed the directory structure for PR templates to house multiple templates
• Renamed the pull_request_template.md to default.md

Review Points

• Please share your thoughts about these changes

[kotaaaa]

Thank you! Let it try!

[nick-y-ito]

@kotaaaa Thank you!