Document the Ingest V2 write path

[fmassot]

Summary

• document the native Ingest V2 request path from REST to the ingester WAL
• describe control-plane shard creation, replication, and indexing assignment RPCs
• explain shard acquisition, WAL fetch, split publication, checkpoint fencing, and cleanup
• clarify commit modes, durability boundaries, Chitchat coordination, and retry semantics

Why

The architecture documentation did not have a current end-to-end view of the write path. This adds a single code-linked reference for the data path and the control-plane, ingester, indexer, and metastore interactions.

Validation

• reviewed against the current router, ingester, control-plane, indexing, and PostgreSQL metastore implementations
• verified all 26 relative documentation links resolve
• git diff --cached --check