Add clear guidance on dark-mode and light-mode asset usage

### Overview
The Base Brand Kit includes multiple logo and asset variants, but it is not always clear which assets should be used in light-mode vs dark-mode contexts. Explicit guidance would help teams apply the brand consistently across different UI themes.

### Problem Description
Developers and designers may be unsure about:
- which logo variants are intended for dark backgrounds
- whether color inversion is allowed or discouraged
- how to choose correct assets for light/dark UI themes
- consistent usage across web, mobile, and marketing materials

This can lead to inconsistent or incorrect brand presentation.

### Expected Behavior
- Consumers should easily understand which assets to use in light and dark contexts.
- Brand usage should remain consistent across themes and platforms.
- Asset selection should not require guesswork.

### Steps to Reproduce
1. Open the Base Brand Kit repository.
2. Browse available logo and icon assets.
3. Attempt to choose assets for both light and dark UI themes.
4. Notice the lack of explicit guidance on correct usage.

### Proposed Solution
- Add a documentation section titled “Light & Dark Mode Usage”.
- Clearly specify:
- which logo variants are intended for light backgrounds
- which variants are intended for dark backgrounds
- any restrictions on color modification or inversion
- Include simple visual examples demonstrating correct usage.