Adds defineSimpleRetriever and docs.

[mbleigh]

Building out my demo I found it kind of annoying to write a retriever because converting things into Documents is tedious. This provides a "simple retriever" that makes it very easy to transform arbitrary data into documents. For example, here is the Data Connect retriever from my demo implemented as a simple retriever:

export const destinationRetriever = defineSimpleRetriever(
  {
    name: 'destinationRetriever',
    configSchema: z.object({ limit: z.number() }),
    content: 'knownFor',
  },
  async (query, { limit }) => {
    const result = await searchDestinations(dc, {
      query: query.text(),
      limit: limit,
    });
    return result.data.destinations;
  },
);

Pretty nice, right?

[pavelgj]

This is neat and I think it should exist, just wondering if it should exist here... the more I think about it the more I think it's probably OK that it's exported via @genkit-ai/ai/retriever... not sure, just saying it out loud... feel free to ignore.

[mbleigh]

I think it's the right place for it, but fair to ask.