Add a gallery example showing individual basic geometric symbols

[michaelgrund]

Description of proposed changes

As mentioned in #1193 so far we have no gallery example in which all basic symbols are shown. This PR adds such an example to the gallery.

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to docstrings or tutorials.

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

[core-man]

Great, the example will be very helpful. I have one question: shall we use a for loop instead of plot those symbols individually? This will make the code much shorter, e.g., the Line styles example.

[michaelgrund]

Great, the example will be very helpful. I have one question: shall we use a for loop instead of plot those symbols individually? This will make the code much shorter, e.g., the Line styles example.

Also thought about that, however, with all the 14 symbols I'm not sure if this would make the code more complex to read. What do the others think about it?

[seisman]

Great, the example will be very helpful. I have one question: shall we use a for loop instead of plot those symbols individually? This will make the code much shorter, e.g., the Line styles example.

I think the current script is good.

[seisman]

Just a few tiny suggestions. Otherwise, the PR looks good to me.

[michaelgrund]

Just a few tiny suggestions. Otherwise, the PR looks good to me.

Thanks for the review @seisman. Unfortunately, removed the units during the replcament actions.

[core-man]

This PR looks great to me. Only one minor comment above, but it's also okay for me to not use quotes.