Update coding guide with some basics on branching in shader code

[ggetz]

Description

I went down a bit of a rabbit hole today on this topic, and figured it'd be useful to document this for people like me who do not have a formal education in graphics. Based on my understanding from those more knowledgeable than I:

• Added a few basics around branching in shader code and how it could potentially affect performance
• Cleaned up some minor formatting issues in the GLSL section of the guide

Issue number and link

N/A

Testing plan

N/A — Review the guide content.

Author checklist

• I have submitted a Contributor License Agreement
• I have added my name to CONTRIBUTORS.md
• I have updated CHANGES.md with a short summary of my change
• I have added or updated unit tests to ensure consistent code coverage
• I have updated the inline documentation, and included code examples where relevant
• I have performed a self-review of my code

[github-actions]

Thank you for the pull request, @ggetz!

✅ We can confirm we have a CLA on file for you.

[ggetz]

@jjhembd Would you mind taking a quick pass on this when you have the time?