DOC/projections: Make example design consistent

[yvonnefroehlich]

Description of proposed changes

This PR updates the projection examples to use the same colors for land and water within in one category.
So far I have used this color combinations:

If we have agreed on the colors, we can use different ways to passe the colors (GMT name, RGB code, HEX codes).
There are also some changes to make the code consistent.

Preview:

• Old: https://www.pygmt.org/dev/projections/index.html
• New: https://pygmt-dev--3464.org.readthedocs.build/en/3464/projections/index.html

Fixes #3458

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 command is:

• /format: automatically format and lint the code

[seisman]

Looks good to me.