doc: Use plural for units for the other quantities

[yvonnefroehlich]

Description of proposed changes

This PR is similar to PR #2251 and changes the units for other quanities to plural.

• documentation
• datasets
• tests
• examples
  • gallery
  • tutorials
  • get started
  • projections

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

[yvonnefroehlich]

I am not sure about the changes in rose.py (please see commits 9113543 and 76171ba). If singular is correct or at least sounds better, I am happy to reverse these changes.

[seisman]

Since we may not reach a consensus about some changes and the changes are all minor, I prefer to delay this PR to v0.9.0 instead.

[seisman]

Ping @GenericMappingTools/pygmt-maintainers to give this PR a final review before merging.