docs: improve PatchCore backbone layer documentation

[rashidrao-pk]

📝 Description

This PR improves the PatchCore documentation by adding guidance on backbone selection and intermediate feature layer configuration.

🛠️ Fixes #1144

The update includes:

• Example usage of different backbones.
• Example layer configurations for commonly used backbones.
• Clarification that valid layer names depend on the selected backbone architecture.

This should help users avoid configuration errors when switching from the default PatchCore backbone.

✨ Changes

Select what type of change your PR is:

• 🚀 New feature (non-breaking change which adds functionality)
• 🐞 Bug fix (non-breaking change which fixes an issue)
• 🔄 Refactor (non-breaking change which refactors the code base)
• ⚡ Performance improvements
• 🎨 Style changes (code style/formatting)
• 🧪 Tests (adding/modifying tests)
• 📚 Documentation update
• 📦 Build system changes
• 🚧 CI/CD configuration
• 🔧 Chore (general maintenance)
• 🔒 Security update
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)

✅ Checklist

Before you submit your pull request, please make sure you have completed the following steps:

• 📚 I have made the necessary updates to the documentation (if applicable).
• 🧪 I have written tests that support my changes and prove that my fix is effective or my feature works (if applicable).
• 🏷️ My PR title follows conventional commit format.