Tutorial "Plotting data points": Add legend for size coding

[yvonnefroehlich]

Description of proposed changes

So far we do not add a legend for the size-coding in the tutorial "Plotting data points". This PR extends the tutorial by adding such a legend via an io.StringIO object. There is also issue #3675 by @maxrjones suggesting to add an example showing how to create a legend for size-coding, as this can not be done automatically in GMT.

Fixes #

Preview: https://pygmt-dev--4214.org.readthedocs.build/en/4214/tutorials/basics/plot.html

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[seisman]

Looks good to me.

If I understand it correctly, issue #3675 can be closed?

[yvonnefroehlich]

If I understand it correctly, issue #3675 can be closed?

As PyGMT allows creating a legend via an io.StringIO object, I think including a size-coding legend in this tutorial is fine, and no separate example is needed.