add examples of using full text search in mongodb integration

[MetroCat69]

from the docs it is unclear that you can use full text search with the document store. We should change the docs to give an example on how to do so.

[bilgeyucel]

Hi @MetroCat69, thanks for the feedback. We have separate docs pages for each component MongoDBAtlasDocumentStore, MongoDBAtlasFullTextRetriever and MongoDBAtlasEmbeddingRetriever. I have updated the integration page to reflect the full text search support and linked the docs pages: PR link