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

[yvonnefroehlich]

Description of proposed changes

This PR aims to improve the width default for pen in the documentation of the supplements. It is changed from default to 0.25p. Changing this also in the supplements was unfortunately overlooked in PR #7074 - sorry!

In case it is welcome, I can change the pen default in "coupe.rst", "meca.rst", and "polar.rst" to the form [Defaults: width = 0.25p, color = black, style = solid] for the sake of consistency.

Related to

• Use "0.25p" instead of "default" for default of pen width in documentation #7074
• 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!