Update README for the "Intro to PyGMT"

[yvonnefroehlich]

Description of proposed changes

This PR aims to expand and update the README for the "Intro to PyGMT".
I would move the note to use a Jupyter notebook from the first tutorial to the README of the intro. Actually, I am wondering whether we still need this note after PR #2662. What do others think? I'm happy to receive feedback 🙂.

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

[seisman]

The tutorial's recommended method is to use a notebook, and the code will be for a notebook environment.

Before PR #529, Figure.show() only works in Jupyter Notebooks. In Python scripts, users must use Figure.show(method="external") instead. That's why we have a similar note in PyGMT v0.3.0 documentation (see https://www.pygmt.org/v0.3.0/tutorials/first-figure.html).

After PR #529, this note no longer makes sense. Users can run the examples in any environments they like (Python console, Python script, Jupyter notebooks or any others).

So I think we should remove this sentence.