DOC: Mention datetime tutorial in Cartesian linear projection

[yvonnefroehlich]

Description of proposed changes

Besides referring to the upstream GMT documentation, it makes sense to mention the PyGMT tutorial regarding how date time input data and format can be visualized in PyGMT.

Also adjust the line length to 81 characters.

Preview:

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]

It's better to write the gmt-docs role in one line, like:

:gmt-docs:`Geographic coordinates <reference/coordinate-transformations.html#geographic-coordinates>`

[yvonnefroehlich]

Hm. I just tried this in commit 864579e, but the style checks fail (locally and on GitHub) because of the too long line.

[seisman]

OK. please revert the changes.

[yvonnefroehlich]

Done in commit 7de29df.