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

A coherent docs-only change that deletes the RocksDB backend guide, redirects its URL, and standardizes node/snapshot/statesync/Giga guidance on PebbleDB; internal links and the redirect anchor check out. No blockers — the main note is that a useful PebbleDB-specific section was deleted along with the RocksDB page and has no new home.