Add project keywords to the pyproject.toml file

[michaelgrund]

Description of proposed changes

Addresses #2314 and adds keywords to pyproject.toml file.

Further suggestions are welcome!

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.

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

[willschlitzer]

Should "geology" be on the list of keywords or is that too generic?

[yvonnefroehlich]

Should "geology" be on the list of keywords or is that too generic?

I am fine with "geology" being on the list of keywords. Fronts (see gallery example Line fronts) and the possibility to use self-created symbols can also be useful for crafting maps in geology. Furthermore, the gallery example for the Ternary diagram is also related to geology.