docs(multilingual): add pg_search backend to BM25 selector and comparison table

[r266-tech]

Drift catch from #1755: pg_search shipped as the 5th BM25 backend in v0.7.0 and was already added to retrieval.md (the "Five pluggable BM25 backends" table) and configuration.md (env-var description + tokenizer enum). developer/multilingual.md was not updated and still enumerates only 4 backends, so a user reading the multilingual page to pick a BM25 backend for their bank's language doesn't see pg_search at all — which is the only Citus-compatible option and the one with the richest tokenizer set for CJK / mixed scripts.

This adds:

1. Backend selector bullet (L196): extends (native, vchord, pg_textsearch, or pgroonga) → (native, vchord, pg_textsearch, pgroonga, or pg_search).
2. Comparison table row (after pgroonga): the pg_search row, mirroring retrieval.md framing (Citus-compatible, configurable tokenizer) and configuration.md tokenizer enum (chinese_compatible, jieba, chinese_lindera, etc.).

Both surfaces (hindsight-docs/docs/developer/multilingual.md + skills/hindsight-docs/references/developer/multilingual.md) have the identical drift; both files are last touched on 2026-05-26 by #1538 (commit cb04cb7) ~13 min before #1755 landed pg_search (commit 4cd260b). The v0.7.1 release sync commit picked up versioned_docs/version-0.7/ automatically per the release process, so versioned snapshots are intentionally out of scope here.

If this is already covered by a separate patch, feel free to close and mark accordingly.