Add sphinx extension myst-nb to enable writing executable Markdown notebooks

[seisman]

Description of proposed changes

This PR adds the Sphinx extension myst-nb to enable executing code cells in a file and insert the output directly into the file. The file format can be (1) ReST; (2) Plain Markdown; (3) Jupyter Notebooks; (4) MyST text-based notebooks. See https://myst-nb.readthedocs.io/en/latest/authoring/basics.html for more details.

The text-based MyST notebook is the most appealing format. To convert a plain Markdown file into a MyST notebook, we just need to do two things:

1. Add metadata at the top of a markdown file. The simplest example is:

---
file_format: mystnb
---

2. Write codes using the following syntax

```{code-cell}
a = "This is some"
b = "Python code!"
print(f"{a} {b}")
```

This PR also updates the install.md file as an example showing how the extension works. In the installation guides, we provide a test script and include an external image (added in #3249) to show how the output looks like. It's not ideal since the image may be outdated when we update the test script. The myst-nb extension can execute the codes and directly insert the output image below the codes, so the codes and the image are always consistent.

Preview: https://pygmt-dev--3379.org.readthedocs.build/en/3379/install.html#testing-your-install

Notes:

1. The myst-parser extension is removed because it's required by myst-nb and is automatically enabled by myst-nb.
2. The myst-nb extension is necessary for writing documentation that needs images. For example, an image is needed to explain justification codes well.
3. This extension can't replace sphinx-gallery now, but maybe it can in the future (xref Enable thumbnail galleries of notebooks executablebooks/MyST-NB#396)
4. myst-nb is one of the subprojects of The Executable Books Project. We're also using their sphinx-copybutton and sphinx-design extensions. Maybe one day we should consider migrating to their Jupyter Book project, which is built on top of Sphinx and a collection of Sphinx extensions.

[seisman]

Add metadata at the top so that myst-nb can recognize it as a MyST notebook.

[seisman]

---
tags: [remove-stdout]
---

These lines are cell-level metadata for controlling the format of the output. Usually it's not necessary. Here remove-stdout is necessary to hide the long output from pygmt.show_versions().

[seisman]

No need to include the external image.

[weiji14]

Can these be more specific to point to a specific folder instead of using **?

[seisman]

Done in 90bb110.

[weiji14]

These lines are cell-level metadata for controlling the format of the output. Usually it's not necessary. Here remove-stdout is necessary to hide the long output from pygmt.show_versions().

Would it be nicer to actually show the output of pygmt.show_versions() actually? Suggestion to:

1. Split into two code blocks, one for pygmt.show_versions() and one for fig = pygmt.Figure() below
2. Collapse the output of pygmt.show_versions() in a dropdown using the hide-output tag (see https://myst-nb.readthedocs.io/en/v0.13.2/use/hiding.html#hide-code-cells).

[seisman]

Done in 609d759 and 7a38068 and looks better.

[weiji14]

Readthedocs is showing the PyGMT version as v0.1.dev64+g7a38068.d20240812, but I guess that's ok 😆