docs: add comprehensive product card merging guide with financial analytics

Add bilingual documentation (EN/RU) for Wildberries product card merging:

- Detailed guide on merged cards (склейки) creation and management
- Analytics tracking for merged product variants
- Advertising traffic distribution model across variants
- Financial analytics with 4 cost allocation methodologies:
  * Proportional to orders (unit-based allocation)
  * Proportional to revenue (value-based allocation)
  * Equal distribution across variants
  * Advertised variant only (conservative approach)
- Complete TypeScript implementations with examples
- Strategic recommendations and best practices
- Troubleshooting section

Updated guide indexes to include new product management section.

Files added:
- docs/guides/product-card-merging.md (English)
- docs/ru/guides/product-card-merging.md (Russian)

Files modified:
- docs/guides/index.md
- docs/ru/guides/index.md

Author: salacoste

docs/guides/index.md

@@ -19,6 +19,9 @@ In-depth guides for production deployment and advanced SDK usage.
 
 ## Module Guides
 
+### Product Management
+- **[Product Card Merging & Analytics](/guides/product-card-merging)** - Merged cards (склейки), advertising traffic distribution, and cross-variant analytics
+
 ### Advertising & Marketing
 - **[Promotion & Advertising](/guides/promotion-advertising)** - Campaign management, bids, budget, and analytics