Add GitHub Discussion templates with emphasis on reproducible examples

[Copilot]

This PR adds comprehensive GitHub Discussion templates to address the lack of reproducible examples in community discussions. The templates guide users to provide detailed information that makes it easier for the community to offer effective help.

What's Added

The new .github/DISCUSSION_TEMPLATE/ directory contains five specialized templates:

1. General Question - For straightforward questions about DeePMD-kit usage and concepts
2. Usage Help - For detailed troubleshooting with strong emphasis on reproducible examples
3. Ideas and Suggestions - For proposing new features and improvements
4. Show and Tell - For sharing projects and achievements using DeePMD-kit
5. Potential Bug Discussion - For discussing possible bugs before creating formal issues

Key Features

Each template is designed to encourage comprehensive information sharing:

• Reproducible Examples: Templates include detailed sections for input files, configurations, commands, and error messages
• Version Information: Structured fields for DeePMD-kit version, backend details, and installation method
• Clear Structure: Consistent with existing issue templates using GitHub's YAML form syntax
• Appropriate Labels: Each template applies relevant labels for proper categorization

The templates particularly address the core issue by including sections like:

- type: textarea
  id: reproducible-example
  attributes:
    label: Reproducible Example
    description: |
      **This section is crucial for getting effective help!**
      
      Please provide a complete, minimal example that others can use to reproduce your issue...

Example Impact

Before: "My training isn't working, what's wrong?"

After: Users are guided to provide:

• Complete input.json configuration
• Dataset structure and sample files
• Exact commands run
• Full error messages
• System environment details

This structured approach will significantly improve the quality of discussions and make it easier for community members to provide targeted assistance.

Fixes #4506.

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