Investigate the utility of Embedding Templates

[anakin87]

We should experiment to see if for retrieval purposes it is beneficial to use Embedding Templates such as the following:

This article talks about {{doc.meta["company"]}}, it was published on {{doc.meta["publication_date"]}}. Here is the article's content: {{doc.content}}
(from #5390 (comment))

There are mixed opinions about them, which should be clarified by experimentation.
A related discussion can be found in #5745, in particular in #5745 (review).

[sjrl]

I'm wondering if the new OutputAdapter in this PR #6936 would be able to accomplish this? I'm not sure if it can handle the input type List[Document] and an output type of List[Document]. Do you happen to know @vblagoje ?