Improve docstring test legibility for junior devs

[pushfoo]

Changes

tl;dr Improves the state of #1570 by outputting the example docstring spec on failure & generally improving test clarity

• Add a brief explanation of the expected example docstring format to test failures by expanding the relevant function docstring
• Use Path.read_text to immediately close source files after read
• Rearrange functions in dependency order
• Rename variables for clarity
• Add comments

How to test

1. Delete python -m line from any example(s) in arcade/examples or one of its immediate child folders, except for perf_test and internationalization example files / folders.
2. ./make.py test
3. Check for the full docstring of check_code_docstring in the output

Follow-up work

While working on this PR, I discovered the following problems:

1. The GUI examples do not match our docstring standard
2. The GUI examples (arcade.gui.examples.*) are not covered by the test due to the test assuming all examples are in the same folder.
3. Recursion into deeper submodules (arcade.examples.gl.ray_marching.*) is broken

Generalized Docstring Check Functionality?

I do not know what @eruvanos is planning to do with the example structure. The second two problems may be best handled by refactoring the test to:

1. fully recurse modules / folders
2. generalize docstring check functionality into something which takes a callable