Finalize user-defined indexes API and documentation for v26.0 release

[qodo-code-review]

Description

• User-defined indexes API response format changed from map to array to improve consistency and usability
• Index protobuf message now includes a name field for better data completeness
• Comprehensive documentation added for user-defined indexes feature introduced in v26.0
• Installation and upgrade guides updated to reflect v26.0 preview status

Deliverables

• API Schema Update: Convert ListIndexesResponse from map<string, Index> to repeated Index and add name field to Index message
• Service Implementation: Update UserIndexReadsideService to populate name field and convert list response to array format
• Test Updates: Modify all index-related tests to work with array-based responses instead of dictionary lookups
• Documentation: Create comprehensive user-defined indexes guide covering creation, management, querying, and configuration
• Release Notes: Update installation guide with v26.0 preview information and add what's new section

[linear]

DB-1798