docs(db-mongodb): note on indexing localized fields & per-locale growth (#13469)

Closes #13464

Adds a note to the Indexes docs for localized fields:
- Indexing a `localized: true` field creates one index per locale path
(e.g. `slug.en`, `slug.da-dk`), which can grow index count on MongoDB.
- Recommends defining explicit indexes via collection-level `indexes`
for only the locale paths you actually query.
- Includes a concrete example (index `slug.en` only).

Docs-only change.

---------

Co-authored-by: German Jablonski <43938777+GermanJablo@users.noreply.github.com>