Fix GUI example docstrings

[pushfoo]

Changes

Follow-up to #1750, prepares for further improvements to example docstring tests.

• Rename one of the GUI example files
• Add or correct top-level docstrings in GUI examples for clarity
• Ensure all docstrings include an example run command per project standards

How to test

I know the instructions below are a pain. This PR is a step toward fixing the problem.

Locally retarget test_example_docstrings.py by making the following changes:

1. add imnport arcade.gui at the top
2. set `EXAMPLE_ROOT = "arcade.gui.examples"
3. In test_docstrings, replace arcade.examples.__path__[0] with arcade.gui.examples.__path__[0]
4. (Optional) if using PyCharm, make sure the run configuration for this test sets the working directory to the project root
5. Run the test, either through the GUI or with pytest tests/unit/test_example_docstrings.py

[eruvanos]

Great work, thanks!