Change test repo branch names from master to main

[d33bs]

Description

This PR changes references to the master branch to be main instead within testing repositories. This is a step towards inclusive language usage and aligns with software best practices more broadly. Along with this I also updated places within various documentation where master references have been updated to main.

Thanks for any feedback!

Closes #116

What is the nature of your change?

• Content additions or updates (adds or updates content)
• Bug fix (fixes an issue).
• Enhancement (adds functionality).
• Breaking change (these changes would cause existing functionality to not work as expected).

Checklist

Please ensure that all boxes are checked before indicating that this pull request is ready for review.

• I have read the CONTRIBUTING.md guidelines.
• My code follows the style guidelines of this project.
• I have performed a self-review of my own contributions.
• I have commented my content, particularly in hard-to-understand areas.
• I have made corresponding changes to related documentation (outside of book content).
• My changes generate no new warnings.
• New and existing tests pass locally with my changes.
• I have added tests that prove my additions are effective or that my feature works.
• I have deleted all non-relevant text in this pull request template.

[d33bs]

Thanks @gwaybio ! Merging this in.

[d33bs]

Thanks @gwaybio ! Merging this in.

[d33bs]

Thanks for the review on this @falquaddoomi !