Add gallery example showing how to adjust line segment ends (caps and joints)

[yvonnefroehlich]

Description of proposed changes

This PR aims to add a gallery example showing how to adjust line segment ends (caps and joints).

• Add basic code
• Combine to Subplot
• Add docs
• Fix typo in upstream GMT documentation Example GTM_caps: Remove un-needed commas gmt#8315

Orientated on the upstream GMT example: https://docs.generic-mapping-tools.org/dev/reference/features.html#tbl-penex

Preveiw: https://pygmt-dev--3015.org.readthedocs.build/en/3015/gallery/lines/line_segment_ends.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 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.

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[yvonnefroehlich]

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