Add documentation of output formats

[artheus]

Formats:

• json
• json_lines
• json_stream
• gelf
• msgpack

Documentation suggested in #300

[patrick-stephens]

We probably should link out or back to the plugins that support this?

[github-actions[bot]]

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[lecaros]

We need to: a. Add a reference from the plugins that support this, or b. Add this piece of documentation to a visible part of the docs. (summary, main outputs readme, other)