Document user-defined indexes feature for KurrentDB v26.0

[qodo-code-review]

Description

• KurrentDB v26.0 introduces user-defined indexes built on secondary indexes mechanism
• Users need comprehensive documentation on how to enable, manage, and utilize this new feature
• Documentation should cover API endpoints, JavaScript filter/selector functions, and query capabilities
• Existing secondary indexes documentation link needs correction for proper navigation

Deliverables

• Feature Overview: Document user-defined indexes introduction, enabling configuration, and relationship to secondary indexes
• API Management: Document HTTP API endpoints for creating, starting, stopping, deleting, listing, and getting indexes
• Filter/Selectors: Explain JavaScript filter and field selector functions with type constraints and error handling
• Usage Patterns: Document reading, subscribing, and querying user-defined indexes via filtered $all API and Query UI
• Link Correction: Fix configuration guide reference path in secondary indexes documentation

[linear]

DB-1796