Add write_cache markdown documentation

[Copilot]

Creates comprehensive documentation for the write_cache component covering functional capabilities and gRPC service interface.

Documentation Structure

• Functional Capabilities: Extent management, transaction handling (commit/abort), table operations (list/drop/evict), cache eviction strategies, memory management with watermarks, and database operations
• gRPC Interface: Complete service definition with 5 RPC methods (Ping, GetExtents, ListTables, EvictTable, EvictXid), request/response message structures, and data type specifications
• Configuration: RPC settings, storage paths, and memory watermark thresholds
• Architecture: Partitioning strategy, storage hierarchy (in-memory + disk spillover), and concurrency model

Documentation focuses on functional behavior and interfaces without code references or implementation details.

Original prompt

On write_cache, document write cache in markdown file

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[sonarqubecloud]

Please retry analysis of this Pull-Request directly on SonarQube Cloud

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[garthgoodson]

@copilot for the write cache document, update the grpc section so that everything under the specific rpc call (like ping) is indented.