Extend CORS coverage

[stanislav-zeman]

Give more detailed example (diagrams would be nice) and explain usage of the mechanism. Explain used headers and possible related packages such as cors.

This could later be extracted to "Auth" lecture as the "REST API" one is already pretty big.

[tisnik]

ah interesting, I was going to fill-in the same issue ;)

[stanislav-zeman]

Added a diagram in d0b165a. Unsure whether it was sufficient. Will leave the issue open so we can further reflect later.