Add more example code standards

[core-man]

Description of proposed changes

A code standard about string value is added in CONTRIBUTING.md (Originally posted in #983 (comment). See also #983 (comment) and #983 (comment)):

• Double-quotes are used to indicate a string value instead of single-quotes. Example: "05m".
• An argument with a string value set should be added `` on both sides of the expressions. Example: ``style="v"`` results in style="v".

Fixes #983

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 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 commands are:

• /format: automatically format and lint the code
• /test-gmt-dev: run full tests on the latest GMT development version

[maxrjones]

This guideline seems fine to me, but we can leave it open for a bit longer to see if anyone else has other preferences.

[core-man]

1. Python arguments, such as import statements, Boolean expressions, and function arguments should be wrapped as code by using `` on both sides of the code. Example: ``import pygmt`` results in import pygmt.

I find we use both `True` (True) and **True** (True). E.g,

• https://www.pygmt.org/dev/gallery/lines/line_custom_cpt.html?highlight=true
• https://www.pygmt.org/dev/api/generated/pygmt.datasets.load_earth_relief.html?highlight=true

Shall we add an example in the first rule? e.g., ``True`` results in True. This corresponds to Boolean expressions, right?

Meanwhile, shall we move this example in the first rule: ``style="v"`` results in style="v"? This is for function arguments.

[seisman]

Shall we add an example in the first rule? e.g., True results in True. This corresponds to Boolean expressions, right?

Yes.

Meanwhile, shall we move this example in the first rule: style="v" results in style="v"? This is for function arguments.

Yes.