We need to revisit how to trigger the CI checks based on the events (pull_request vs. push), branches (main, staging, and the rest), and matrices (e.g., OS and Python versions).

Potentially we could remove tox, but then it might make sense to enforce the CI tests on every push event for all the branches. Currently CI tests is only enabled for push to main and pull_request for other branches.

We also need to document the branching model and how it affects running the CI checks.