Docs: document CI/CD behavior / implementation

[dreamorosi]

Description of the improvement

Summary of the proposal
With the number of workflows and scripts growing in our CI/CD processes (based on GitHub Actions) there's a need of documenting the relationships between each workflow/script and also the different triggers.

The proposal is to create a .github/workflows/README.md file and have a short explanation + diagram of each major workflow, similar to what done in this comment of #1023.

How, where did you look for information
N/A

Missing or unclear documentation
.github/workflows/README.md

Improvement
Creating the doc would help all existing maintainers to have context on how the workflows. This would also be useful to onboard additional maintainers / collaborators in the future.

Related existing documentation

N/A

Related issues, RFCs

#1023