DOC/tutorial "Plotting lines": General improvements

[yvonnefroehlich]

Description of proposed changes

This PR improves the tutorial "Plotting lines":

• Add missing words
• Improve formulations, add comments within the code examples
• Rewrap the line length
• Split setting up the Cartesian plot (Figure.basemap) and plotting of the line(s) (Figure.plot); I personally feel this is clearer to understand, but I am open to other opinions on this change
• Do not use the pen parameter (change the pen attributes) in the first section, as this parameter will be explained in detail in the second section

Preview: https://pygmt-dev--3587.org.readthedocs.build/en/3587/tutorials/basics/lines.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.
• 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.