Clarify compute shader N-body tutorial

[pushfoo]

Changes

• Add optional star color to make movement visualization easier
• Rephrase tutorial page, replacing "ball" with "star" in Python & shader source to be consistent with the example theme
• Lower default resolution to accommodate smaller screens
• Lower default number of stars to accommodate weak GPUs
• Add top level constant + units in comments on existing top-level constants
• Reorganize code to follow tutorial's structure
• Clarify comments & docstrings
• Update Window title to match top level docstring

Why

This addresses issues I found while reading the code to understand #1782.

How to Test

1. Run ./make.py serve
2. Go to http://localhost:8000/tutorials/compute_shader/index.html
3. Proof read for accuracy
4. Run main.py in the example folder to make sure nothing new is broken, only the movement issues previously reported

Follow-up work

1. Attempt a fix for Mistake in compute shader example #1782
2. (TBD / uncertain) Compact this example to use a more efficient layout in memory?
3. (TBD/ uncertain) Separate into steps like other tutorials?