Add a gallery example for a double y-axes graph

[core-man]

Description of proposed changes

This gallery plots two base maps with the same X-axis but different Y-axes, while it also plots data points and lines for the two corresponding Y values, separately.

Fixes #1009

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.

Slash Commands

You can write slash commands (/command) in the first line of a comment to perform
specific operations. Supported slash commands are:

• /format: automatically format and lint the code
• /test-gmt-dev: run full tests on the latest GMT development version

[core-man]

It's unnecessary to put arguments in separate lines, especially for lines no longer than 88 characters. What I usually do is writing all arguments in one single line, and let black (i.e., run make format) do the formatting automatically.

@seisman re-formated in 65f418b.

[core-man]

Maybe "basemaps" following the comment #876 (comment)

@seisman done in 29dfdaa.

[seisman]

Also need to add the new category to

pygmt/doc/conf.py

Line 74 in b2b3d1f

"../examples/gallery/maps",

.

[core-man]

Also need to add the new category to

pygmt/doc/conf.py

Line 74 in b2b3d1f

"../examples/gallery/maps",

.

done in b58469b.

[seisman]

This PR looks good to me.

Ping @meghanrjones @michaelgrund for final comments and approvals.

Please note that this example is added to a new category "Base maps". Do you think it's good? Also ping @willschlitzer @weiji14.

[michaelgrund]

Except the provided minor suggestions this PR looks good to me. The new category is suitable in my eyes. Good work!

[maxrjones]

Nice! I just have one small recommendation to avoid confusion about stating that two basemaps should be plotted and then plotting three basemaps (common x, y1, y2).