docs: add section for bff swagger

[slettner]

Add a section to the documentation pointing out the use of the swagger docs for sending http requests similar to here: https://docs.jina.ai/fundamentals/flow/access-flow-api/?highlight=swagger#use-swagger-ui-to-send-http-request

Maybe we can also print a reference to the docs at the end of the jina now command. Like "not sure how to interact with the flow? Check out our docs to learn more! "