Add a gallery example for varying transparent points

[seisman]

Description of proposed changes

Add a gallery example for the new feature in #626.

Address #615.

Preview: https://pygmt-qmth2blje.vercel.app/gallery/plot/points-transparency.html

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to docstrings or tutorials.

[seisman]

Ping @weiji14 for a review.

[weiji14]

Sorry, a bit busy right now, so a quick comment instead of a full review. Could we use a simpler straight line example instead of a sine curve here? I.e. like this image in our unit test (but maybe modify it slightly)

[seisman]

The example is rewritten to plot the figure below.