docs: prepare state store guidance for RocksDB removal - #62

Solid, well-scoped deprecation pass: the RocksDB page is removed with a valid redirect (#seidb-configuration exists), the deleted asset has no remaining .mdx references, and PebbleDB guidance is consistent across node setup, snapshot, state sync, and Giga migration. Two operator-facing correctness gaps block: the new snapshot backend-verification command can green-light a RocksDB-containing archive, and the "resync onto PebbleDB via state sync" instruction silently drops the archive-node caveat that the deleted page carried.