Use "0.25p" instead of "default" for default of pen width in documentation

[yvonnefroehlich]

Description of proposed changes

This PR aims to improve the width default for pen in the documentation. It is changed from width = default to width = 0.25p.

Related to

• Expand 3-D basemap properties with back-walls and gridlines #3993
• Use 0.25p iinstead of default when describing pen thickness #4006
• Fix default for pen parameter in API documentation pygmt#2134

Reminders

• Make sure that your code follows our style. Use the other functions/files as a basis.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Describe changes to function behavior and arguments in a comment below the function declaration.
• If adding new functionality, add a detailed description to the documentation and/or an example.

[PaulWessel]

Looks good, thanks.