Docs: Github Actions

### What problem are you facing?
This template contains a couple of Github actions e.g. https://github.com/upbound/upjet-provider-template/blob/main/.github/workflows/ci.yml which are not self-explanatory. But they are triggered automatically when users fork this repo. 

### How could Upbound help solve your problem?
Some basic documentations on how to use and adapt the Github actions. 
