CI: Require at least one code block separator for example files in the Style Checks workflow

[seisman]

Description of proposed changes

Add a step in the Style Checks workflow, to ensure all example files have at least one code block separator # %%.

Address #2805 (comment).

See https://github.com/GenericMappingTools/pygmt/actions/runs/6864172516/job/18665360359?pr=2810. If I revert the changes in PR #2805, the workflow fails with the following error:

Code block separator '# %%' is required in following example files:
examples/gallery/maps/choropleth_map.py

which means the added step works as expected.

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 wrapping a new module, open a 'Wrap new GMT module' issue and submit reasonably-sized PRs.
• If adding new functionality, add an example to docstrings or tutorials.
• Use underscores (not hyphens) in names of Python files and directories.

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

[weiji14]

Also mention this # %% rule in https://github.com/GenericMappingTools/pygmt/blob/v0.10.0/doc/contributing.md#code-style, and link to #2660?

[seisman]

Also mention this # %% rule in https://github.com/GenericMappingTools/pygmt/blob/v0.10.0/doc/contributing.md#code-style, and link to #2660?

Done in 8752ef2.

[weiji14]

One minor spelling issue, otherwise ok. Might be good to have someone else proofread the documentation.

[seisman]

@yvonnefroehlich Maybe you can give this PR another review?