Add MongoDB Table Engine to `named_collection` page.

[thomoco]

MongoDB not specifically mentioned on named_collection page: https://clickhouse.com/docs/en/operations/named-collections

However, seems to be included in code: https://github.com/ClickHouse/ClickHouse/blob/3f959848bd6631f0a86e06d7a3eb9bfc1177a8d2/src/Storages/StorageMongoDB.cpp#L178

Tangential questions:

• what other table functions and table engines are named_collection supported on?
• is named_collection supported on all table functions and engines?