PlantUML guide: Add tip about

[damithc]

Add the tip mentioned in nus-cs2103-AY2223S1/forum#384 to the https://se-education.org/guides/tutorials/plantUml.html

[mfjkri]

Will make a PR for this!
PR will create a new header in the Tips and Tricks section for using reference frames in PlantUML.

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It would look like:

Making reference frames

We can split UML diagrams to break complicated diagrams into simpler ones.

group sd reference_frame_name

end

[damithc]

@mfjkri Go ahead and send a PR.