POC: Add common docstrings for meca/coupe

[michaelgrund]

Description of proposed changes

Related to #3817.

Preview:

https://pygmt-dev--3825.org.readthedocs.build/en/3825/api/generated/pygmt.Figure.meca.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.

Slash Commands

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

• /format: automatically format and lint the code

[michaelgrund]

/format

[michaelgrund]

Closed as the issue will be addressed in another format (see #3829)