Resolve "Adjust pipeline to default branch name change"

Summary

• Since we changed from 'master' to 'main' we have to adjust the pipeline

Explain your context

• 'master' is now 'main' as it's the newer default

Please list breaking changes, new features or bugfixes

• Breaking changes:

• New features:

• Bugfixes:

  • Fix CI pipeline and readthedocs generation by updating default branch name

Closes #251 (closed)