Add gallery example "Scale bar"

[yvonnefroehlich]

Description of proposed changes

This PR aims to add a gallery example showing how to configurate a scale bar:

• list of available modifiers
• code examples
• seperate PR Figure.coast: Add alias "box" for "-F" #2823 (similar to pygmt.Figure.basemap)

Upstream GMT documentation:

• https://docs.generic-mapping-tools.org/dev/basemap.html#l
• https://docs.generic-mapping-tools.org/dev/coast.html#l

Preview: https://pygmt-dev--2822.org.readthedocs.build/en/2822/gallery/embellishments/scalebar.html

Fixes #2627

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

[yvonnefroehlich]

/format

[michaelgrund]

Suggested change

	# only to the code within the "with" block)
	# only to the code within the "with" statement)

[yvonnefroehlich]

Hm. I am still not 100 % sure whether within the "with" statement sounds better than within the "with" block and makes sense. Maybe we can write under the "with" statement if we want to use statement instead of block?

[seisman]

I feel 'within the "with" statement' is better.

[yvonnefroehlich]

OK. Should I also update the formulations in the Figure.inset line example:

pygmt/pygmt/src/inset.py

Lines 119 to 131 in 06ae818

	>>> # Use a "with" statement to initialize the inset context manager
	>>> # Setting the position to top left and a width of 3.5 centimeters
	>>> with fig.inset(position="jTL+w3.5c+o0.2c", margin=0, box="+pgreen"):
	... # Map elements under the "with" statement are plotted in the inset
	... fig.coast(
	... region="g",
	... projection="G47/-20/3.5c",
	... land="gray",
	... water="white",
	... dcw="MG+gred",
	... )
	...
	>>> # Map elements outside the "with" block are plotted in the main figure