Docs: Better explain difference between frontend and backend plugins [UXR]

[josmperez]

Incorporate feedback such as:

• Early in the docs, before users start their journey, make sure that difference between frontend and backend plugins is clear. Explain that developers who want to have alerts need to have a backend plugin.
• User wants to understand better what’s going on on the backend. He says plugin was “black box” without clear explanation for why it is the way it is.
• Wanted to better know whether to use frontend or backend, and role of dataframe in selecting one or the other. Must understand that dataframe must connect to the SQL connector, and once that works, then it’s easier. If SQL connection doesn’t work, nothing happens because dataframe is missing.
• Could benefit from more examples about backend use. He learned from backend tutorial, but example is not so detailed.